here's some information ive gathered,
with 7 different spring classes all with different weights and free arches, I feel this is even more complicated then just a +1 -1 issue. but that's just me.
Ended up being just a faulty connection on the back of the cluster. Swapped whole screw plug with one of the unused ones off of the side infocenter. And its worked fine ever since.
theres a place in upstate NY that repairs sending units and/or will build a brand new one for you. doesn't charge too badly too. repairs anywhere from $70-300 on average.
you should be able to swap the electronic gear and the mechanical gear no problem in the tcase. as for fuel sending units yes, its vendor mis representation of thinking MJs are XJs.
1. no known full sending unit replacement available. but you can replace just the pump with a generic if necessary.
2. not reproduced either to my knowledge, gotta find a junkyard pre-91 xj/mj or upgrade to a 91+header
these are ebay ones. they work pretty good. but the passanger side ISNT a "objects in mirror are closer then they appear" mirror so getting used to the 1:1 ratio on that side can be a little strange.