A diffrent kind of transmission problem!

[amc401]

Ok. So I have been trying to get this thing to do better on MPG than my V8 Chevy, and it ain't happening!!! I have a 06 Xterra with 97k on her. I bought it at arround 90k and the only maintenance I can go on from previous owner is from carfax. And this thing was well maintained!! Lately I have noticed that the truck will not shift into OD until the truck is fully warmed up. And that can take about 7 miles in the mornings. Way to long in my opinion. Now this is my first car out of about 40 that's not a GM, but all my others go into OD at same time weather its cold or warm. I have a warranty and took it into the dealer and they don't know if its a problem or if its normal. But they agree it seams wrong. What help hu!!! They decided to wait till they get a Xterra in stock to test drive it to see if it does the same. Good idea, but if the other ones does the same and it IS a problem they still won't know. So for all thoes out there I was wondering if yalls do the same thing. Caus if not, I have a problem and this is so wasting my gas!! Oh and also the fluid is good and the radiator fluid is not mixing. No codes and no info of problems of on dealer scanner.