My Thanks to THEFERMANATOR As I copied and placed his similar post here.
I have decisions to make and I have no idea what a custom PROM is?
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91-93 used old style wiring harness, and a different valve body due to a different lockup frequency and eliminiated teh PWM cleaning cycle for 94. 94 is a one year only trans in that it still used the shift shaft for old style mechanical shift linkage, had no bosses for a NSBU, but the new style wiring harness and lockup setup in the valve body. It should also be noted that 91-93 with factory PROMS can not use the newer 94+ lockup solenoid, but uses it's own style(455MHZ VS the 805 of the newer style. Don't quote me on the exact frequencys off the top of my head). 95-96 is just like 94 except they have bosses for the NSBU and the newer style shift shaft for a shift cable instead of mechanical shift linkage. 97 started the rear oiler as it is called where they enlarged the fittings for the cooler lines and made it so one was in the front and one was in the rear. Starting late 99 GM upgraded them again with a solid input shaft, a 1/4" wider band, and a few other internal changes to improve oiling. There were also other running changes through the years to correct problems and failure prone areas, and I'm sure many other things I'm forgetting right now.
Also a 94 trans will fit into any 94 without issue, but cannot be used in a newer truck without finding some way to modify it to accept a NSBU. A 95-96 trans can be used in a 94-96 plug and play so long as you use the correct shift shaft. A 97+ can go into a 96- with afapter lines and fittings and using the correct shift shaft. If you put a 94+ into a 91-93, you either have to use the 91-93 valve body, or get a custom PROM for your controller for the new style lockup solenoid.
