What is the stock tire size for a diesel? The 4.87 gear ration here in the state's was for auto's with 31's. If teh stock tires size for deisel and 4.87's was 28's, it is unlikely you would want to run anything bigger than the 31's you have been running. If diesel isn't cheaper than gasoline, you might consider swapping in a 22r, you could then run 33's(w/o a susp. lift), and have 28's and 4.10's performace and mileage, though w/o the torque ft's you now have with the deisel.

Are there HP mods you can do for the Toy diesel, i.e., turbo's, propane injection etc. If you can bump the performance to better match the tires, you could probably run bigger tires. You could also regear to 5.29's.