With your high mileage, it is not unreasonable to replace all 4 shocks. Alot of people like the Koni Raids or an external reservoir expensive shock. I have used the OEM Bilsteins for G500 and they have held up well on thousands of mles of washboard roads. If your springs are not broken in the rear (check them carefully) I would raise your truck using a 30mm or 50mm polyurethane spring spacer. 50mm will give you two inches, maybe more if your shocks are bad. You need to pull off the shocks to install a spring spacer anyways. I have the 30mm version. Spring spacers will raise the Gwagen body off of the axles but will not inprove your diff clearance, aka actual ground clearance.
To raise the axle, you need bigger tires. The easiest and almost as good as best option is a 33" tire on the original 18" rims. I recommend a 275/70/18 AT tire. From a stock 30.5" tire, a 33" tire will get you an additional 1.25" of both diff and body clearance. If you do this in conjunction with a 30mm spacer as I have, you will end up with almost 3" of "lift." No rubbing with this tire size either.
Higher center of gravity will effect your ESP and make it potentially much more more active. This is a problem if you routinely drive tight turns on your commute.