You can eliminate the disconnecting axle and vac. system all together if ya want.
Go to the parts store and get a set of differential inner seals and a pass. side long shaft from a newer XJ.
You'll have to remove the D30's gear/diff set and then drive the seal into the pass side of the housing - just as well do the driver's side while you're in there. Re-install the gears, slide the shafts back in and add fluid. Then you can just make a cover for the vacc. disconnect housing and seal that up.
Much cheaper probably, but a bit more work.