I replaced my old 4l80E with a used one I bought off of the internet the transmission upshifts and downshifts a little rough but otherwise drives fine. When I put it in reverse the truck will walk itself backwards but pushing the gas does absolutely nothing, it doesn't rev up nor does the speed increase, it seems as if it is in limp mode only while in reverse any ideas? I have a 1994 k3500 with a 6.5 TD.

 

Did you ever find the problem? Mine does the same thing sometimes

 

Reverse and 2nd are the same if both are sluggish it is time for a rebuild or replacement sorry to break the bad news but that is what it sounds like.