A while back a bought a 1994 Chevy c2500 the transmission was good would shift fine never did anything wrong. One day I drove to dmv and I went to back out and it wouldn’t reverse so I stopped checked the transmission fluid it was dark but not black and it was full so I kept going I had stuff to do so I was on the way back and then it wouldn’t go into 3rd, then 2nd, and then 1st stopped working. Now if it sits for a while it will go into reverse or first and will drive for roughly a minute then will go completely out. Would love to get some insight on what it could be (never seen it go out like this before) if it’s as simple as fluid and filter swap or if I need rebuild/swap transmission