Duramax does not rev over 2100 rpm???? does not shift properly can not get into overdrive unless it is a steep downhill grade.

You need an auto-electrician with a pro grade scan tool computer to find out which module and sensors has failed.

Worn out Transaxle

Have to rebuild it or install a reman

it sounds like its is shifting strange it could be the torque converter or something inside the tranny making it shift sooner than wanted.

A failed transmission causes this.

check vacuum hose to and vacuum actuator for the transmission

Hi:
It is not a problem to turn Overdrive off, as it is recommended that you turn it off when hauling a load or decending a steep grade, as you get improved power and engine braking. The down side of not using overdrive is that your engine will rev somewhat higher, and your fuel economy will be less.

Should still be under warranty. Take to dealer for repair and/or adjustment.

bad transmission

There is an O/D button on the dash or on the shift lever. The "O/D" stand for overdrive the vehicle is normally in the overdrive mode unless you turn it "off" with the O/D button. The only times that you should have to turn it off is. Towing or hauling a heavy load or mountain driving to have better power for pulling the steep grades and to have engine braking affect coming down the mountain. If you driving in the mountains remember to use your engine to hold your speed down, not brakes you'll burn the brakes up in no time. Read the owners manual for mountain driving. Keep in mind everytime you turn the vehicle off and restart it, it's going to go back to the O/D mode.

d will do just fine but most say city driving use d3 but at higher speeds you will burn more gas because the car will rev higher