I've been in the same predicament with mine. I bought a used np231 from a '97 wrangler on Craigslist for $75 only to find out the input shaft splines are different on those compared to the YJs. Plus I took it apart and found a bunch of broken pieces in it too. My next option is to buy a master rebuild for $200 online and rebuild it myself. Ive now learned from experience that you probably shouldn't trust buying a used TC. Think about it, why are they selling their TC anyways? My seller gave me a great reason, I believed him, and he got my money.
So, spend a little more than twice the amount and get an almost new TC or $75 and run into problems soon.