Automatic trans trouble

I have a 1992 4WD Chevy PU truck with a V-6. It has 200,000 miles on it. It would not shift out of low gear. I took it to a repair facility for an opinion, and they thought it might be a piece of dirt obstructing a passage. Short of doing a tear down they could not be certain. It had been this way for a few days. Ironicly on my trip home from the facility, the trouble came clear. I dropped the pan and the fluid looked like chocolate milk, and the filter had seen better days.I think their original diagnosis may have been correct. What steps should I take to clean this up before I try to reassemble, other than a new filter screen, pan gasket and fresh fluid? The truck runs good other than that. Does anyone have any advise?

Other than fresh fluid and filter like you said, it would be advisable to flush the tranny aswell to get rid of that bad fluid.

And this might mean aa bigger problem.

So how did that work out for you?
Seems to me that only going into 2nd gear is the limp home mode too on most cars. Oftne that's something like a corroded electrical connector.

We put new trans fluid and a bottle of trans cleaner in and drove it a little. At first it was stuck in low gear again, but a full throttle run blew out whatever was causing trouble. It runs fine now. I hope it stays that way.