As already said they've made the Jeep GC's in the 3.0L diesels for years. Ours gets 25mpg doing 75 to 80 on the freeway all-day long, if you baby it you can squeak near 30 out of it on the freeway. Bad thing is at 80 it spings 3k rpm, they've needed another gear for years. 5th+ will do this thing wonders in MPGs.
The Liberty's sucked because they were excessively underpowered and plagued with EGR fouling.