To answer your question, the common 4WD hubs appeared first on the mid-92 MJ models, not the 91 models. So your easiest option is to grab a used 2WD or 4WD XJ front axle (whichever you prefer) from a 93 and up instead of trying to convert your existing 2WD front axle. I have to do the same thing one of these days on my 91 2WD............