46re acts like its in forward and reverse at same time

This is not an a-body related question, but it seems like the tech knowledge on this site about anything Mopar is the best you can find on the web. My son has a 2000 Ram 4x4 with a 5.9
and a 46RE. The truck will not drive forward or reverse and will not even roll with the trans shifted to neutral. The engine is definitely under a load when trying forward or reverse. It will roll with the transfer case shifted into neutral. When shifting the trans into reverse or drive, it feels like a normal shift into gear. I'm hoping that we might get lucky and the problem could just be a stuck shift valve but I've got very little experience with trannys. Hoping one of you gurus can give me an idea what to look at.