it's not quite "mopar" but perhaps consider a Volkswagon 1.9 TDI?

There was someone who has swapped one into a later jeep with much sucess. Near 30mpg iirc.

1979 Dodge Lil' Red Express - 360 rwhp, 13.2 @ 103mph
1968 Coronet: 318, 2.76, 15.2 @ 92mph! (SOLD)
1976 Valiant: 360, 3.90, 12.90 @ 106 (SOLD)
1989 Shelby CSX #500/500