4l80e nightmare
4l80e nightmare 
Burnt clutches, or maybe cut seal bleeding pressure. I had a bad seal after a rebuild and had to rev the hell out of it to get it to move. It is all just guessing until you pull it apart though.
Worn boost valve causeing high pressure and blew out the forward piston seals.
Or the forward clutch cushion spring broke and is jammed up in the drum.
Do a pressure test it will tell you if it's hydrualic or mechanical.
Or the forward clutch cushion spring broke and is jammed up in the drum.
Do a pressure test it will tell you if it's hydrualic or mechanical.