I have a 2000 Silverado 1500 with the 5.3. The trans went out and I replaced it with a good used transmission (watched it drive into the shop) out of a wrecked 2003 Silverado. I installed a new torque converter (pre filled before install) and new filter.

Filled the trans before starting and then topped off after fluid level dropped. When I went to run it through the gears to make sure all the air was out of the system it wouldn’t engage. It will pull down the engine RPMs in all gears but won’t turn the wheels.

I checked for pump flow at the trans cooler and double checked the fluid level. Both are good.

truck will roll in neutral and park will hold it. I tried switching the transfer case through all positions to make sure it was engaged and still no movement.

Im at a loss as to what might be causing issues.

Any help is appreciated.