Given that the frictions of the immediate run up was caused by mercantile policy (in the standard telling, at least), what caused the agricultural towns to unite with urban merchants?
As you rightfully point out, the origins of the resistance against the perceived overreach of the British government during the imperial crisis were based in the urban centres of New England where the different acts passed after 1764 had a greater impact. Now, it does need to be said that acts such as the Stamp Act did have an impact outside of Boston but not to the same extent. If we look at the simply question why anyone would rise up against the crown during the time period in question, we would find a wide variety of motivations in urban centres alone. Urban merchants had their own reasons for resistance against the crown while those of the "lesser sorts", whether it be waterfront workers, shoemakers, or apprentices had others that can't be ignored or discounted. Women and other oppressed minorities such as slaves of African or Native American descent had their own agency which they used not only to resist (or support) the crown, but also to resist oppressive structures in their immediate midst.
A good place to start with this inquiry is in the Boston Pamphlet and the creation of committees of correspondence. The Boston Pamphlet comes during a time of increased protest against the crown after two years of moderate hiatus after the Boston Massacre of 1770. For the sake of this answer, we will briefly focus on section B and C of the pamphlet. Section B is an overview of the perceived injustices by the British Parliament towards the people of Massachusetts. Predictably, the first two points in section B makes the claim that Britain is both passing laws and is imposing taxes without the consent of the people of Massachusetts. However, it was point 6 which not only prompted the creation of the Boston Pamphlet but which gives us an insight into what sort of question actually mattered to people outside of Boston: "Britain has upset the governmental balance of power by usurping the legislature’s authority to pay the salaries of the governor, lieutenant governor, and judges." In other words, the British Parliament has prevented the colonial assembly to perform its traditional role of paying the salaries of important officials (and thus making them answerable to the colonial assembly and therefore the people). This, the pamphlet states, "will, if accomplish'd, complete our Slavery." If the colonial assembly doesn't pay the salary for these important officials, including the governor and important judges, what would stop them (the officials) from being held answerable to the people and not take direct orders from Parliament? What would stop them from depriving men from their property, as in point 12 which states that Britain "has violated colonists’ property rights by granting to others land developed by settlers in the past, and by redefining colonial boundaries, thus forcing landowners at great inconvenience to recertify their holdings."
Section C, "A Letter of Correspondence to the Other Towns", suggests the creation of Committees of Correspondence throughout the Massachusetts Bay Province so that Boston could "communicate with our fellow Sufferers" about "this recent Instance of Oppression, as well as the many other Violations of our Rights under which we have groaned for several Years pasts." The Boston Pamphlet created a popular response in town meetings throughout the province. Although only men with property could attend these meetings, they made up a big portion of men in rural Massachusetts seeing as many owned farms which were seen as a symbol of self-determination. Hundreds of Massachusetts town answered in the affirmative by creating their own Committees of Correspondence. Lexington for example answered with a statement that stated that they looked "upon themselves in common with their Brethern and Fellow Subjects throughout the Province, to be greatly Injured and Oppressed in Various Instances by Measures of Government lately adopted."
1772 was only the beginning but it established a line of communication between the urban center of Boston with the greater Massachusetts countryside. The crucial year, however, is 1774. Historian Ray Raphael is the scholar who has really explored this particular period out of the perspective of the Massachusetts countryside and I would refer to his book (see sources) for a more in-depth explanation. During the lead-up to the Boston Tea Party, there were already tea boycotts and protests against the unconsented taxation in the Massachusetts countryside. the After the infamous Boston Tea Party in December 1773, the British Parliament introduced a series of acts (The Coercive Acts or, as they were known by some, the Intolerable Acts). Although the Boston Port Act is arguably the most well-known of these, it was the Massachusetts Government Act which would play a vital role in putting the Massachusetts countryside aflame. The Massachusetts Government Act did not only practically revoke the Massachusetts Bay Charter but also deprived the people of Massachusetts from selecting town representatives (who would be selected by the governor and who would also appoint other important officials like judges) and from holding impromptu town meetings (that now required pre-approval by the governor).
Due to the large amount of enfranchised men in the Massachusetts countryside, 90 % in Worcester alone, this meant that a large portion of Massachusetts outside of Boston were effectively disenfranchised. This was not taken lightly by these rural towns. By October 1774, courts throughout the Massachusetts countryside had been forcibly closed down by the town militias. Citizens used intimidation tactics previously used in Boston to force officials to rescind their positions. In the Powder Alarm of September 1774, General Thomas Gage (also governor of Massachusetts) removed the provincial powder that was stored in the Powder House, in modern-day Somerville, in an expedition that triggered alarms throughout Massachusetts and led to the mustering of several militias that all marched towards Boston in belief that the British troops had fired upon colonists. The task of General Gage was to neutralize a rebellion before it happened. His attempt failed. By December 1774, the Massachusetts countryside had taken the initiative away from Boston and were now the leading the resistance against Great Britain. As laid out by many resolves by county conventions throughout Massachusetts, including the more famous Suffolk Resolves, rural Massachusetts were also preparing for war. If Gage once again sent out troops to impose the Coercive Acts on the people of Massachusetts, the men were ready to defend their self-determination through war.
Primary sources:
Secondary sources:
The Revolutionary Worlds of Lexington and Concord Compared by Mary Babson Fuhrer in The New England Quarterly, Vol. 85, No. 1 (March 2012), pp. 78-118.
The First American Revolution: Before Lexington and Concord by Ray Raphael (The New Press, 2002).
The Road to Concord: How Four Stolen Cannon Ignited the Revolutionary War by J.L. Bell (Westholme Publishing, 2016).
The Minutemen and Their World by Robert A. Gross (Hill and Wang, 1976 [2011])
The Glorious Cause: The American Revolution, 1763-1789 by Robert Middlekauff (Oxford University Press, revised 2007 ed.)