This question has multiple parts that I'll try to address.

1. Did the U.S. Army have any units titles as 'grenadiers?' The short answer to that is no. No American infantry regiments were ever called grenadiers in the regular army but there were some militia companies that used the term in their names. Let me be clear about the term 'grenadier' even by the time of the American Revolution the term was almost entirely a ceremonial one. Specialized infantry regiments trained in throwing grenades had fallen out of fashion by that time because early grenades were notoriously unreliable and limited to as far as a man could throw. This tactic did not work well as armies began to solely operate using firepower with bayonets and swords as auxiliary weapons. During this period names like the British Grenadiers or Napoleon's famed Grenadiers a cheval were purely ceremonial and designed to convey that these men were more elite than their counterparts without the name.

2. Why didn't the US utilize grenades more frequently prior to WW1? Again this comes down to how battles were being fought during the 19th century. With the advent of mass produced rifles weapons battles became even more about firepower and less about shock (such as bayonet charges or cavalry charges). If the man you're fighting is almost never going to get within a hundred yards of you why should your government go through the expense of providing you with an unreliable weapon you'll almost certainly never, ever have the chance to use?

3. Grenade use during the Civil War. Now it is true the grenade saw a bit of s resurgence during the American Civil War. Inventors during this time were always coming up with new novelties that either the Union or Rebel governments might purchase as weapons. One such weapon was the Ketchum grenade which looked like a football with a large tail on one end and a metal 'plunger' on the other. The idea was a man would throw this grenade towards the enemy, it would land on the 'plunger' mechanism that triggers the explosion, and those around it would be injured by shrapnel. Well it didn't work all that well and during the siege of Vicksburg there was reported that when a Union soldier threw one of these grenades over the rebel lines, the rebels would catch them in large things like tablecloths and sheets in order to avert triggering the plunger (which more likely than not wouldn't go off anyway) and then throw them right back over towards the Union lines!

During the 50 years between the U.S. Civil War and the Great War grenade technology improved significantly and changing tactics that saw men started to see close range action more and more often. These two changes in warfare are what made the grenade become a common part of a soldiers kit and a useful anti-personnel weapon. I'm on mobile right now so I can't effectively cure the books I'm basing this off of, but when I get home I'll edit this post to include them. Cheers!