40mph vibration

[LandCruisinMy93]

I know a vibration is pretty much impossible to diagnose via the 'net but I'll try anyway. I drove my truck home from CT on well worn Michelin tires and it was smooth up to about 75mph. I had my new Cooper Discoverer ATPs installed a week or two ago and now intermittently I have a strange vibration around 40mph. The whole truck vibrates like its having a seizure. The only thing that changed was tires. I went from 265/75 to 285/75. The tire shop said there weren't any issues balancing the wheels. Any idea where to start w/ this? The obvious solution is to drive faster or slower than 40 but that is the ideal speed for back roads around me.
Thanks gents

Mine does the same thing, Of course nothing on the front is stock anymore and im on 40's but it still vibrates at 40, not at 35 and not at 50 but right between 40 and 45 ish..

 

[Offroads4x4]

Check your wheel bearings.
Just a thought.

 

[Slamfire94]

Mine does the same thing, Of course nothing on the front is stock anymore and im on 40's but it still vibrates at 40, not at 35 and not at 50 but right between 40 and 45 ish..

did you ever find out what this is?

 

[Malleus]

My '95 does this at 55 mph. I just don't drive that speed anymore.

 

[Slamfire94]

My '95 does this at 55 mph. I just don't drive that speed anymore.