you dont have to have a trick rear diff, you can put the bigger diff/shafts/beam in from a 2wd in the back and it'll more than handle the abuse. The ratios are 0.02 out IIRC which is nothing to split hairs over.

Why spend £1000 on a diff when you can pout a full 2wd back axle in for 1/3 of that?

Cheers
Dan