a 4bt or NPR would be the best bet. The only thing that concerns me is if either motor would be too tall for the Durango because it is IFS. I don't think that you could pull it off without a lift. A 3/4 ton diesel (ie 6bt, powerjoke, or Duramax) wouldn't really give you enough mileage to justify the work. I am dam certain though that if you put in a 4BT you would get at least 25mpg. Lee Alessi (on this board) is getting 24mpg with 38's and a 6400 lb Kaiser M715, which has the aerodynamics of a refrigerator. Hope this helps. This board is decent for 4BT info, but you should also check out pirate4x4 for additional 4BT info.