Something to consider is that the AX15 should be a direct replacement, you lose a 6th gear but rebuilt AX15's are under $1200. The AX15, NV3550, and NSG370 are usually considered all equally strong.
If you do get a replacement transmission I will pay you core charge value for your old 6spd.
Nathan
*edit*
After looking into swapping it into my XJ it has no durabilty advantage over my AX15, and very little gear advantage, with the known compatibility issues, CPS, t-case clocking, shift lever, and knowing that there will be un-known swap issues it would cost me minimum $1200 to rebuild and install the 6spd in my XJ (not counting the cost of the "core" transmission). It will cost me about $500 to rebuild my AX15. Even I don't want a 6spd that bad. (that cost difference almost covers a Teraflex Dual Rate swaybar which is a better use of the money)
Nathan