d30 is worth rebuilding if you're planning on staying with 33" tires. Consider upgrading the axleshafts to '95+ due to the larger ujoints.
I upgraded my 231 with the following, on the cheap:
Wide chain and sprocket kit
6-gear planetary
both can be found in a 231c or 231d or 233c.
Super-short SYE
2lo kit
4:1 is not worth it in the 231, it has pretty low RPM limitations. Otherwise it's a good transfer case worthy of upgrading.