My 84 bolted up fine, and you get a 10% reduction in high and low gears. So basically it matches the 4:1 kits in high range. But most of the T-Cases are not compatible with the after market gear kits.
So if you have one, you might as well use it to bring your 29-30" tires back into stock range. But I would hold onto your old T-Case for future upgrades. Besides, it's nice to have a spare.