Someone is speaking shit. There is nothing strange about R31 diffs. You can get open centres, LSDs and spools for them, and they can of course be welded. The only odditty is that the axles actually thin out a little after the splines so it's difficult if not impossible to shorten the axles by a small amount.
Back in the day I looked into an R31 rear end, and it looked like a decent centre (Kaaz do one), 4.1 Pintara ratio and R31 axles are a good combo.
Hen
Also note that if you have a factory silo LSD, the chances it still works and isn't totally flogged are about the same as happening to pick up a super-model at a female weight lifting contest.