I bought a 1991 GMC Jimmy 2Wheel Drive Automatic transmission. I drove it home about 70 Miles and parked it.

I knew going into this transaction that Reverse was inoperative but didn't really care.  I I put a new Battery and a Serpentine belt on.  When I started it on Monday I let it warm up while putting all my equipment for the days work inside which took about 45 minutes.  It drove to work and home with no troubles which put about 100 miles onto it.  Tuesday morning I started it up and drove off.  I got about 3 miles down the road and it started slowing down going up a short hill.  I made it to the top of said hill, turned around and got about one mile back toward home and would not go any further.  Upon examination I discovered that even after running for almost an hour the transmission fluid is still way over full by about a full quart.  Suggestions, please?