Idon't really know much about the early pickups other than they are the same as the chevy LUV's. I will suggest what I believe to be true. The rear axle is probably an Isuzu Corporate 10bolt. Its probably an identicale unit as whats in the rear of an early trooper (drum brake models prior to about 1988). If you wanted rear discs, a swap to a larger 12 bolt or possibly a d44 could be done.
I can not say what it will take to hook the drive shaft up.

However it may be that you simply need a new pinion bearing and seal. Which could cost more than a junkyard axle, but worth checking into.

Its possible that a front third member from any later 4wd isuzu with a 10 bolt front diff may bolt in if you can't find a whole rear axle.

Hopefully someone can verify that any of this is true or false. It seems reasonable, but I'm not totally sure, so double check first.

one link worth checking might be Chevy Luv's

Good Luck,
Mike W