I am planning to write a fictional account of a judicial duel in the late 11th century. I'm still not sure in which area I would like to set the piece. The piece will not feature the actual duel, but I would like to focus on relationships within and between the two supporting "posses". I would like the piece to take place right before news has spread about Pope Urban ii speech in 1095. The themes and intrigues of the narrative is not necessary to discus here, but I would like to know where and how (even if) such an event was possible. I understand that I will have to consult a lot of texts before tackling something like this, so even a point in the right direction of sources will be greatly appreciated.
I assume you are referring to judicial ordeals (trial by fire, by water, by combat, etc). The go-to book on various judicial ordeals is Robert Bartlett's Trial by Fire and Water: The Medieval Judicial Ordeal (Oxford, 1988, 2014). Trials by ordeal were used in place of testimony and evidence and was not the same as resolving a 'blood feud'. Yes, trial by combat was still used to settle questions of legal guilt in some cases in the 11th century (God was thought to have sided with the righteous party); in fact, even some monasteries in the French Vendomois in the 11th century opted for trial by combat using a champion in their place to settle legal claims - this was specifically proscribed by the papacy in the 12th century, and then at 1215 the Fourth Lateran Council forbid any clergy from participating in any trial by any ordeal whatsoever. The 13th century saw the transformation of European legal systems into that which we would recognize today - evidence based trial processes...filled with lawyers.
In terms of 11th century disputes, a very interesting paper is Stephen White's "Pactum... Legem Vincit et Amor Judicium" - The Settlement of Disputes by Compromise in Eleventh-Century Western France, The American Journal of Legal History, Vol. 22, No. 4 (Oct. 1978), pp. 281-308.
If you search Google Books you'll find lots of references and examples of trial by combat.