I have a HST JD 4100 4wd. This idea is for one attached to the FEL. Kinda like the ditch ones..I saw one overseas on a HST and it did very well in hard soil..
unless one has creeper gearing in the tractor will not work very well you will over whelm the trencher,
I have a add on trencher made by auburn trenchers, they sold a transmission that one would split the tractor and add, (I did not have the tractor it come off of thus not a very good set up, made them for jeeps as well,
I did set up a creeper drive with some extra wheels but I can not get enough traction to move both the trencher and the tractor at times,
video of a old a Auburn trencher on a ford 2000, looks like hard rocky ground,
Ford 2000 Auburn trencher - YouTube