the rear MJ shaft is like a foot longer than an XJ shaft.
-double check your rear pinion angle (losing a shaft is quite rare and leads me to think something else might be wrong)
-measure for a new shaft using the method on Tom's site
-buy new shaft
-wait patently
-install new shaft
-go have fun.
I can't seem to find an image of it, but sealing the rear of the t-case is easy. just wrap a few 8" lengths over the end to seal it, then wrap the roll around and around to keep it all there.