Completely agree and the vss sensor not working could cause it not to shift as well. I need to go over these items and I bet it works.Limp mode is usually lack of a 12v signal to the transmission, my vote is something electrical - some issue with the plug to the transmission. Do you have the old tranny plug to compare pins? I suppose you could google a pic and compare.
I'd find this hard to believe also, if it worked fine before the terminator, I would think base tune has to be close if not exactly stock for the transmission.

I will give it a shot
