I bought a lower mile 2006 4l80e and swapped from 2wd output shaft to 4wd output shaft, with the new output shaft, rear endplay was very tight, I barely got any movement on the dial indicator, maybe .001. Front Endplay is .010

At the time I also threw in Borg Warner HE's, Sonnax boost valve, and Transgo HD2.

The issue I'm trying to resolve is, when the Trans is in 1st/low it feels like you have 10k pounds behind the truck or the parking brake is applied, Reverse, 2, 3, and 4 all feel much better, but I don't think quite right.

I tried brake stalling the engine and the truck struggles to rev even on dirt or gravel.

Is the really tight rear endplay the cause of this?

I just pulled it back out and everything looks clean, all bushings/bearings look new, fluid is free of any metal. I'm not sure which direction to go, could this be an issue with the Transgo plate? I did get the newer style plate.