The Ax15 is the smoothest operating one
Buddy of mine has a tj with the nv3550, at idle it sounds like a couple marbles rolling around and only from 1st to 2nd it has a weird grinding noise, he has to put it into neutral for a split second then put it into 2nd to shift smooth
For piece of mind I'd go new, if it's junkyard or Craigslist I wouldn't pay over $200...