Due to he popularity Toyota trucks had at the height of the wheeling craze, most wrecking yards are still charging a premium for their t-cases. If you keep an eye out on craigslist, and the For sale section here and on Pirate, you can usually find top shift cases for under $150 and forward shift cases(which can be converted) for under $100.
Used 5.29 diffs should run about $350 w/open diffs, $500 w/an auto-locker, or about $750 w/an ARB or a Toyota e-locker. To help offset the cost of the swap, you should be able to get about $500 for your set of 4.56's from someone only looking to run 31's or 33's.
While not an alternative to dual cases, I found kicking on the a/c would put enough drag on the engine that I could descend hills and ledges w/o the need to use the brakes, and turning up the idle screw on the intake plenum helped to reduce stalls when trying to climb up rocks and ledges. Years ago on a run I ran into a guy who used a hand throttle to free up his right foot so he could ride the brakes, similar to what can be accomplished with an automatic transmission.
