I have a 02 Dmax short box crew (Just bought...under warrantee) with a high speed vibration from 76mph to 83-4mph. The new factory tires(take offs from a new one) have been rebalanced(the rear ones) & the driveshaft & ujoints have also been replaced with no change. The sound appears to be coming from the rear.(we think) At about 80 it's at it's worst, you can feel it in the peddles, floorboard & seats, but not really in the steering wheel. It sound like the worlds largest womens vibrator at that point(it will give your a headache if you continue at that speed) I have Westin tube steps on it that make it all the more worse( like tuning forks) I loosened them up & helped, but it still vibrated just not as loud. These steps just bring out the the problem even more. Could this really be in front end?...maybe the airdam? This is a at speed issue because running in a lower gear(going slower) but in the same RPM range...it does not vibrate. ANY IDEAS???

Did you check the wheels for trueness? If the wheels are not true, a slight wobble can cause a vibration at a certain speed.

UPDATE!! Changed out the driveshaft & ujoints & problem solved.

UPDATE!! Changed out the driveshaft & ujoints & problem solved.
that was what I was planning to suggest . . . .maybe I am learnin more about truck plroblems as I get older . . . .:D