With only 58K miles (before the lift and tires) I broke 5th gear and it locked the shifter into neutral and wouldn't come out. I had to have it towed home. I pulled the tranny myself and took it to a shop. They diagnosed it, replaced 5th gear and replaced all the seals. It only set me back $300 and a few hours of work.
One big difference is the number of miles. Have you considered upgrading to a NV4500? Some time in the future I plan to upgrade to a NV4500, but I have other priorities for now. I know a lot of guys upgrade to a NV3550 out of a newer TJ, stronger then the AX-15, but it is still a medium duty transmission.