You have a lot more miles on yours than my wife's Nitro. I have been inside the timing cover due to a water pump leak. I have not heard any terrible stories about the 4.0, at least not like all the ones about the Durango/Dakota 4.7L. The key to longevity with these engines is making sure that they never ever overheat. My Durango warped a head at 220,000. Luckily, I got to the 4.0 before any damage was done. Before installing the used engine, replace the water pump and timing belt as getting them in and out with the engine installed is a pain. Doable, but a pain.
2014 Ram 1500 Laramie, 73 Cuda
Previous mopars: 62 Valiant, 65 Fury III, 68 Fury III, 72 Satellite, 74 Satellite, 89 Acclaim, 98 Caravan, 2003 Durango
Only previous Non-Mopar: Schwinn Tornado