With the tires basically new and you like how the truck works now, it should cost you under $500 to swap to 4.56's. There is a rear 3rd listed on Craigslist(Gold Country)for $150. It is located in Penn Valley which is about 80 miles from Woodland. You can also find 4.56 geared 3rds and IFS sections on car-parts.com starting out at around $100.

If you have a basic set of tools and jack stands, you can save yourself a few hundred in labor costs. The front section can be swapped w/o removing the front wheels, and you only need two jack stands to do the rear. You should be able to do it in under 4 hours. A second set of hands is helpful for the front, but it can be done with just one.

Yes for $7k you could likely get into a newer Taco, they don't seem to be holding their value as well as the older generations, might be a built in the U.S. issue. or perhaps just more were sold w/o the 25% import tax as with the older gens. However with a new used car you don't know it like you do your '91, you know everything you have repaired/replaced, and what you did to your '91 you may have to do to the new one.

If you aren't planning on lifting the truck to run bigger than 32's in the future, I think you should be pretty happy with the 4.56's and 30's, given the other performance upgrades you have done. A cheap way to find out would be to pick up the rear 4.56 3rd listed(if still available and actually 4.56 gears), drop it in, and see how the truck feels. If it still feels under powered, than you know you would want to go with 4.88's, or maybe a newer rig.