XJ springs are shorter in length and using them brings it's own set of issues to overcome. You'll have to retain your MJ mains to locate the axle and you'll need to properly support it. So if you took a full XJ pack, cut the eyes off, and slapped that under an MJ main, you might have something. being made entirely of used leafs there's no exact prediction about the lift involved.
I've got no idea what an XJ pack is rated for in weight carrying capacity.