Easiest and cheapest is gonna be the #21 code. The computer relies on accurate engine temperature to regulate how much fuel goes in. I'd fix that one first. I know the Mass air sensors on the v6's aren't cheap so maybe a junk yard unit would be best to try to fix that one. You'll probably see a pretty big fuel mileage jump just fixing the temp sensor problem though.

-Todd
1987 Montero 2.6l 5-speed (Sold...but I still miss her)
2008 Jeep Patriot Sport 4x4