2003 Chevrolet Tahoe Transmission

All of a sudden won't stay in gear after running thru carwash.I can cut on and it will go in drive and reverse for a few second/ minutes etc. Then back to neutral so cut it off and keep doing the same things. It was making a flapping/ weird noise last week but stopped. Don't know if that was part of it or not. Hoping it's an easy fix like a fuse or relay or something. Not sure but the ac might not be working right either. It starts up good / I was thinking maybe battery but now not. Let me know if any ideas asap. Wife is having to drive my truck.I have no wheels. Thanx so much. Out of work too. Bad situation.
The change oil light stays on periodically but that is part of the computer thing. It can wait a bit.I have put some new oil in when it goes down some. And the trans fluid leaks a little on the drive and have to add that as well. Have no idea what to do. God help us. Thanx again