Is there a difference in the differential or overdrive gear ratios between 2000 and 2001? someone i met suggested he was getting 20 mpg with his 2001 deisel 4x4 at 70 mph. I have to do 60 to get 20 mpg.

Overdrive ratios are the same, but gear ratios could be different depending on what options you have. You're probably geared a little lower.