OK, that gives us a place to start, as parts availabilty is different depending on truck size. As far as gear choice, there is only 1 gearset available new, and that's a 3.38. All of the bearings, seals, and gaskets are avalable new. Most likly, you have a 3.90 gear, which isn't really too bad.
Swapping the entire rear would really depend on what you plan to do with the truck. Most guys that do a rearend swap are also looking to change the wheel bolt pattern, and perhaps go to disk brakes, in addition to a gear swap. With the larger trucks, it's also a matter of no availabilty of aftermarket gears.
Others will have to chime in on a frame swap, as mine is still riding on the stock chassis.
