I think you'll be happy with that inline 6. They are one of the great American engines that I wish were still made. An inline 6 is one of the few engine designs that is inherently balanced. They are very durable and have great low end torque, which translates into a good power-plant for a 4WD rig.
Stolen from Wikipedia, "The straight-six layout is the simplest engine layout that possesses both primary and secondary mechanical engine balance, resulting in much less vibration than engines with fewer cylinders."
Also shamelessly stolen from Wikipedia, "An inline six engine is in perfect primary and secondary mechanical balance, without the use of a balance shaft. The engine is in primary couple balance because the front and rear trio of cylinders are mirror images, and the pistons move in pairs (but of course, 360° out of phase and on different strokes of the 4-stroke cycle). That is, piston #1 mirrors #6, #2 mirrors #5, and #3 mirrors #4, largely eliminating the polar rocking motion that would otherwise result. Secondary imbalance is avoided because the crankshaft has six crank throws arranged in three planes offset at 120°. The result is that the secondary forces that are caused by differences from purely sinusoidal motion sum to zero".
And one last bit of plagiarism from our friends at the wiki:
"The last in the line of the AMC inline sixes, the 4.0 is regarded as one of the best 4x4 off-road engines. A Motor Trend long-term test of a 1997 Cherokee XJ noted "this long-lived OHV powerplant has a reputation for getting people where they need to go" as well as "much love expressed by owners for the torquey 4.0-liter/190-horsepower inline six." The engine is known for longevity and to go more than 300,000 miles (482,803 km) without rebuilding."