If the axles are cheap enough, then it might be ok for now. But yeah, 3.55s are not enough gearing. I would start planning for the next phase to include 4.56 or 4.88s and A better rear axle (29 spline 8.25, Explorer 8.8 or MJ/XJ Dana 44)
Also, just in case you didn't know, the perches on the XJ rear axle will need to be moved inwards a bit to match the MJ leaf locations.