Welcome to the forum.
I would assume the MO you are looking at was a highway cruiser in order for it to have clocked 122,000 miles over just two years. That's good. Highway driving yields a lot less wear than city driving. Do you know the previous owner?
The VQ engine on the MO is known for its longevity if taken care of. 2-300,000 miles is not unheard of if the engine is properly maintained.
The areas that you should look at are the seals on the engine, transmission, and transfer case. They are prone to leakage. Have a reputable mechanic give the MO a once-over, paying particular attention to the seals, and go from there.
If the MO is cosmetically fine, and the mechanic finds nothing major out of place, then $3,900 sounds like a great deal.
2005 SL AWD, Platinum
-OEM HID conversion & HID fogs
-OEM iPod interface
-Upgraded (non-Bose) speakers - Pioneer TS-A1702C & TS-G1643R
-Muth signal mirrors
-Valley Industries hitch and wiring harness
-Aluminum fuel tank shields