[JaySVX]

With time, an amateur can fix anything. All you have to do is keep taking things off until you find what your looking for. If I were you, and you have a spare car to drive for the time being, I'd put the thing on jack stands, and take off the the transmission and crack it open, see if you can find what's going on. If you can't figure it out, take pictures and put them up here and folks will have a jab at it.

Note: The following is pure speculation, based on no actual knowledge, talking completely out of my ass, and the results could widely range from being dead on to not being able to be further from the actual solution.

In the forward gears you have a high speed clutch and a low speed clutch, and them going can cause slipping in their respected gears. So one might assume that there would be a reverse clutch as well, which , if bad, would cause slipping when in reverse. Like a clutch in a manual trans, if a clutch is not under heavy load, it can still function. However, under load, it's slipping is going to increase, and functionality decrease, thus making heavy acceleration with a bad clutch similar to going up hill backwards in reverse.