From the Union Provost Marshal Papers-Two or More Civilians on the Missouri State Archives website, F1658 pg. 832-838 is a letter (which I think you have seen) Wilkinson to Baker May 25,1865 which gives a description of Robert (Bob) Highley and includes: "Was wounded in one of his legs but does not limp was shot in Illinois at Prairie De Rouge (Prairie du Rocher). Also the Belleville (Illinois) Democrat of April 23, 1864 contained a reprinted article from the Chester (Illinois) Picket Guard which gives an account of the Prairie du Rocher Raid and includes "son of Hiley was shot through the hand and leg." This latter source does not mention Bob Highley by name, however.