I wouldn't fret about buying a used 4wd. Why, as previously mentioned most are rarely used much in 4wd. The drive units are relatively simple. If you drive a used truck in 4wd and it makes odd noises, walk away. I also think if a persons is going to beat his truck, there'll be obvious signs such as dents, messed up interior, debris in the under carriage, etc. If there is service records available, its likely the person didn't beat the truck.
These are just my opinions.
2000 Cherokee 29BH with 6V batteries, LED lights & 400 watts of solar power, flipped axles and raised. 2007 Tundra 5.7L DC-LB with lots of mods. C-co, 8/158th AVN Maint.