The LSD from an 88 Supra will do just fine. ZUK did mine actually. And no, you DO NOT NEED to cut your axle shaft. Mine is a narrow '85, my neighbors was a wider '86. Neither one required shortening. Here's ZUK's write-up on mine.

Note the bearing cup difference on one side. That's all.

ZUK's LSD install


85 4Runner - With NEW Marlin 4.7 Gears!! It's a whole new beast!