First off -
Buying anything used that has 4wd is risky business, I don't care what anyone tells you. 4wd is a VERY VERY VERY expensive component in any vehicle. I currently own a 2008 Jeep Wrangler, I previously Owned 3 jeep wranglers, I partake in serious rock crawling 4wd trips, and I've taken several people here on them. I'll tell you personally, 4wd can be your friend, or it can be your enemy. A used vehicle with 4wd can and more than likely has been used off-roading at some point or another either carrying trailers, pulling some sort of load or just the off the beaten path user. Either way no one buys a 4WD to never use it. What determines your personal 4WD experience is how the person before you took care of the truck. You can do all the Carfax reports you want, you can verify that the person before you was 50 years old. Doesn't mean anything.
BEFORE YOU BUY a used 4WD, you need to take it to a shop, and have it looked completely over. You need it completely inspected for any issues that may arise. A good 4wd shop ( ive been to hundreds over the East Coast) will ask to drive it on a off-road test track, theres one specifically off Hwy 20 going towards I-75 on the right hand side. They will listen to the noises, the power loses or gains and they can make sure its working properly.
I've had serious work done to mine over the last 2 years, and one thing Southern Offroad did for me was put it up on a lift and engaged it to make sure everything was working right.
Don't buy a clunker man. Price a front and rear differential and a transfer case. Then realize most shops will charge you between 3-4 grand for a repair that serious.
My main point in this: Buy smart or buy new. Don't just lay $2500 on a the table and expect to not get problems. My brother knows all to well what that got him on 2 back to back 4wd vehicles.