You need some type of scan tool so you can see what gear is being commanded and work through all of those codes. Start with the lowest numerical code and work your way up. Throwing parts at a problem is not the best way to go about fixing this. If I haven't lost this is the 3rd trans you have installed with the same result. Get a scanner and a service manual and work through the codes.

 

I totally agree and this is the 2nd trans. I do have an Innova 5160 scanner that even has live data. This ECU is from a 99 silverado and the scanner is claiming the ECU doesn't support live data meanwhile the ECU is throwing 40+ codes on the scanner that it will not delete. I can delete them and the scanner will tell me it successfully deleted them but without the engine even running it will tell me it has codes like misfire detected. Now these aren't check engine light style codes...only scan tool can pull these. The codes aren't the same every time either which makes it even harder to understand which ones are valid.

 

If you are getting similar scan codes with the 2nd trans, then obviously the main problem is not in the trans.
In my post #7 I wondered if the trans connector wasn't snapped in fully as it sounded to me like an electrical problem; besides the PCM I would check the trans harness to make sure it didn't melt on an exhaust pipe or got damaged in an earlier trans swap.

BTW - Even though you say "Got a rebuilt transmission and it's doing the same thing" I suspect the new trans works fine in Reverse because even if the PCM commands the lowest line pressure, the boost valve raises it significantly in Reverse.

Sorry you are still having so many problems, but your determination is making you an expert in the 4L60E with a degree from the prestigious "School of Hard Knocks". You''ll be helping others here soon.