Looking for posi rear end for 1985 D250

1st determine what rearend you have,prob a 9 1/4,then find a sure grip or limited slip for it's application,using the term posi[chevyterm] will get you some ribbing...it's possible you could have a dana rear,how many lugs does your wheels have?if 8 lug then you have a dana or a spicer type rear,if 5 lug then prob a 9 1/4..after rereading your post and the trk is a 250 you most likely have 8 lugs which rules out the 9 1/4...look up the trk by yr and model to see what you have or chk under hood for description of options[if its still readable] or better yet post a pic of the rear end cover plate