I looked at your public profile. Is it 4wd? If so, temporarily remove the front driveshaft. At the speed your having the problem I'd suggest inspect the rims to make sure they're not bent. Also, have the tires balanced. Inspect tires for trueness. I had a discussion with a company rep from Detroit Locker about a year ago. He mentioned many company complaints are related to tire company tolerance. Just because you have 4 matching tires doesn't mean they're the same. I think he mentioned using the chalk test to find the correct psi for your truck and tire combo. Once all 4 tires at the desired psi mark tires again with chalk and drive in very straight line on flat pavement. Check to see which tires are slightly taller/shorter. Move the most similar tires to the same axle front/rear.
This could be a stretch in your case. Chalk is cheap.
The chalk line for testing psi would cross the tread from inside to outside. Driving slowly on pavement check chalk. If part of the chalk is not rubbing off add/remove air until the chalk line rubs off in a uniform matter. This gives the tire a flat contact surface. For the lawyers-This also doesn't take into account heavy loads that would require higher psi. If in doubt consult your owners manual. If further questions contact your tire and vehicle manufacturer for more information.
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