I'd love to build a 620 rally or desert truck. Sounds awesome.
You may consider a Toyota 4x4 rear axle right out of the gate. Gear ratios are more plentiful and very cheap. Lockers, limited slips, axle shafts, disc brake kits, etc are all relatively inexpensive on a Toyota axle too. Depending on the width you desire, I would probably start by looking at a '87-'95 rear axle. It should be about the right width and it's a lot stronger than the earlier housings. A Tacoma housing can also be used, but they are a bit wider still. Be careful choosing a Tacoma housing as the most prevalent out there has a 8.4" diff, which is harder to find parts for. You want the regular 'ol 8" diff.
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