First off, you would seperate one of the t-cases into its two sections, and make some minor modifications to it. This would become your crawl box. The only shift options this would have is hi and low. Then, using an adapter, you would bolt up your complete t-case, which would still maintain the standard J shift pattern. This gives you 5 forward gears in stock 2hi, 4hi, and 4low, as well as low 2hi, low 4hi, and low 4 low.
2nd, your finally drive ratio is dependent on which gears you are in obviously. To figure out your lowest ratio, it is simply
1st gear x 2.28(stock t-case) x 2.28(second t-case) x axle gears = final gear ratio.
Mine is 3.92 (R150) x2.28 x 2.56 (chain drive t-case from v6) x 5.29 = roughly 121:1
If you had the 4.7 gearset you'd obviously multiply that into the equation instead of one of the 2.28 gears.
It would indeed be slow enough to crawl with 37"s. Marlin used to have a speed calculator on his site, not sure what happened to it, but if you poke around online, you can find one that allows you to input all the variables above, including tire size, to figure out what speed you'd be moving at at any given time.
