I would say if you know for sure the transmission and T-case are new then check out the engine, check the oil and makes sure it is not black or that it does not smell like gas or coolant. Pull a couple plugs and make sure they don't look that bad. Test drive it, make sure it doesn't overheat. The T-case goes for $1400, and the transmission goes for $1700 without the cost of installation, that would more then bring the price down to 15k. So at 19k you’re getting a pretty good deal.
Don't forget to Search first then ask questions later! The loose nut behind the wheel