4T65E Slips in Reverse and 2nd

Just picked up a 2003 Impala last night with a 3.4 in it. I was told it gets a P1811 code before I bought it, and since it wasn't registered, I just bought it with no test drive. Stupid me. After towing it home, I went to back it up over the tow trucks wheel lift arms. When I did, it slipped in reverse. It moves in reverse no prob, but under a moderate load, it slips. On a test drive around the block, 1st worked good, second seemed to slip though, and it would shift right into 3rd (which did not slip). Couldn't get it fast enough to check 4th. Anyone have any ideas? I see why the P1811 is set now!

Update. The car seems to work in reverse fine now. I drove it around and got a P0742 code and P1811. Talk about hard shifts! Here's the problem though. 2nd gear seems like it's missing. I temporarily disconnected the lockup wire at the harness plug so the Converter clutch stays off. Drives ok, but 2nd is a no go. It shifts from 1st to 3rd. I believe something may have broke during the hard shifts with the converter locked up. Any input?