Jaybdaboat

So I used to be self employed, that didn't work anymore and I picked up a part time to be a delivery driver. That same day I left work pulled up to the gas station, put it in park and pumped gas. I started my car, put it in drive, and it went in reverse. I thought I selected the wrong gear and tried to put it in drive. It went into reverse again, the gear selector moved freely and I couldn't select a gear. I just got this transmission from Craigslist in July so I could move to Indianapolis. I only been above 3000 rpms one time since I got it and babied it 90% of the time. My question is what transmission do I upgrade to so I can stop having these problems. I would like to stay around $500-$800 area, might be impossible because I'm going to have to pay for labor. I have a 93 lt1 that had a stock 700r4 behind it. I would love a 4l80e or 6 speed but that's way out of the budget, I'm guessing. Any good suggestions?