As you've no doubt noticed, there aren't many midsize and smaller vehicles with off-road capability. Many manufacturers have given them up in favor of car-based crossovers.
I'm not a big fan of the Cherokee's handling in on-road driving--it feels heavier than other compact SUVs, partly because it is--but for people who really will use the Trailhawk's off-road capabilities there aren't any clear alternatives. When I reviewed the 2015 Outback recently, I compared it to the Cherokee. Reliability has been pretty good so far, but it's too soon to say how they'll hold up over time.
The Nissan Xterra is ancient, inefficient, and about to be discontinued. I think it can be ruled out.
The 4Runner's key strength is durability. Its efficiency isn't awful. So this could still emerge as the winner.
Jeep offers a diesel Grand Cherokee, but these are very expensive.
If you were to consider another pickup, GM will soon offer a diesel in the new Colorado / Canyon.
Since you have the Outback I assume that when you say you need 4L, you need 4L. But if the Outback does okay, might a Forester also do okay? The latest Subarus are much more efficient than earlier ones.