Rubi TJ: I-6 engine, air lockers, 16" wheels, 31" tires
Rubi JK: V6 engine, I believe elec lockers, elec swaybar disconnect, 17" wheels, 32" tires.
When looking to buy any used vehicle, especially a Jeep, crawl around under it to get an idea if it been abused or how much offroad it looks like it has been. Also while crawling around under there check the oil pan to make sure no leaks especially the rear main seal area of oil pan. Also check the rear output shaft of the t-case to see if it has ever leaked as well.
Common wear & tear I would say are the axle U-joints and Driveshaft U-joints and then the next ones would be Hub Assembly depending on tire size thats on the Jeep.
As for price range, check KBB to give you an idea.