(All from memory...) If it's stock, it will be neither. Look for the G80 option code, which is the Eaton locking differential. It will present as an open differential until the wheel speeds between the two rear wheels is sufficient to cause it to lock. The easiest way to confirm is to put one rear wheel on loose gravel and the other on dry pavement and stomp on it. If you get a chirp and some rubber off the one on dry pavement it's a locker.
I recall you can make it lock by jacking the rear and spinning one wheel (backwards?) while holding the other one, but I could be wrong.
The Constitution needs to be re-read, not re-written!
If you can't handle Dr. Seuss, how will you handle real life?
Current oil burners: MB GLK250 BlueTEC, John Deere X758
New ride: MB GLS450 - most stately
Gone but not forgotten: '87 F350 7.3, '93 C2500 6.5, '95 K2500 6.5, '06 K2500HD 6.6, '90 MB 350SDL, Kubota 7510