You need to check that the dipstick is actually reading correctly. You may be low on fluid and not even know it.
This video steps you through the process of calibrating your dipstick.
https://www.youtube.com/watch?v=DBULieFxGsk
What ecu are you using? If the ecu originally controlled a 4l60e its not going to work right on a 4l80e. Its also possible if you sent it out to have the ecu tuned that it might not be tuned correctly.
The trans connector can sometimes be positioned upside down. Try unplugging rotating and reinstalling it.
Jack up the truck put it in neutral and see if the wheels can spin freely. Maybe something is locked up causing the truck not to move.