Pull the transmission drain plug and check the screen that is behind it, mine was clogged full of clutch plate material. Cleaning it allowed the transmission to start working again but not long after that I had to pull the tranny and replace the clutches.

Over time the clutches get brittle and break off the backing plate and they clock up the pickup screen. The plates are easy to replace but getting the tranny out is a chore, I had to pull the cab and the engine out to do mine.