For those of you who have you heart set on a non disco Dana 30, a disco Dana 30 can be made in to a non disco VERY easy.
So if you find a disco with 4.10s, don't pass it up.
The disco housing is stronger too. All you need it s new shaft, and a new seal on the inner tube. You just gut the axle, and remove everything, and put a block off plate over the access opening.
Then install the new solid shaft and you're done.
Any XJ, MJ, TJ front Dana 30 are interchangeable.
Some rotors are different, and some calipers are different.
Some have the older 1/4 ton shafts, some have the better 1/2 ton 297x U joint shafts.
SOME OLD '84-'86 Dana 30s have CV joint shafts with a round rubber boot at the joint- STAY AWAY FROM THESE, unless you plan on swapping out these shafts anyway.
Some are high pinion and newer D30s are low pinion in both TJs and XJs.
High pinion are desirable obviously.
What I've been told is look for an XJ with a 4cylinder and a standard tranny?