How long have you owned the truck? I have seen trucks that have had the tranny just dogged on for 75000 with a good stack of boxes, the truck will then barely shift without going into limp. Then the guys take off the boxes right before they trade the truck in and believe it or not they don't shift terrible. But when you throw a box back on it at 100000 it get's grumpy. I would say it's possible that your trans is just hurting. It shouldn't be if previous owners were responsible.
LBZ Farm Truck when is that last time you had a truck with 100,000 on it?
I agree that 85 HP shouldn't limp it, but with anything used your don't know for sure what your getting.
sdwalleyes if you've owned the truck since it was new and this is your first box it should definetly not be doing this