an AX-15 is an AX-15. the length is constant. if the trans has a thick aluminum plate between the front and rear halves of the case, it's an AX-15,. But if the trans has a thick steel plate between the front and rear halve of the case, it's an AX-5 and that won't work for you at all. they look similar, but the middle plate gives it away (also the bolt pattern for the bellhousing is very different but the bell isn't always there). the 231 found in the old Jeeps comes in unsealed rear (89 through maybe 96?) and sealed rears (anything after that). can't say I'm 100 percent that they are the same overall length or not.
also, your AX-15 is rebuildable.