We ended up rebuilding the AX15. well sort of. We removed it from the YJ, disassembled the tranny and replaced the bearings and most of the syncro's. None of the parts looked bad and the specs checked out with the feeler gauge. Reverse gear was a little chewed up so I cleaned it up with a dremel. All the bearings were good and so were all the gears. Nothing seemed to be wrong with it other than reverse.
So we put it all back together and slapped it in.
Still pops out of first!
We've done some research and found that others have similar issues with AX15's popping out of first gear. Some people have used a longer stud in the first gear shift rail to increase pressure on the rail. We did that to no avail. We've also triple checked the entire shifter assembly including the plastic grommet that attaches to the shifter itself. It looks good and is snug.
It seems that a couple of people have reported that replacing the thrust washers may help resolve the issue.
I think we are gonna buy the $30 worth of thrust washers and repeat the rebuild...