Hi -
We had a 98 Suburban this past summer that also vibrated at higher highway speeds. We went through everything to try to get it fixed. New tires, wheels, new drive line, rebuilt the rear-end etc. It never did get fixed and the dealer ended up taking it back. It had custom wheels on it.
We recently purchased a 99 Tahoe that DID NOT vibrate!!! We put custom wheels on it and it started doing just what the Suburban did. We had the tires re-balance at least 4-5 times, tried a different set of tires and wheels - it still vibrated. Had them put the stock wheels back on it and it doesn't vibrate anymore.
Some things I learned:
1) When putting wheels on a Chevy - be sure you are only getting HUB Centric wheels. These are made to fit on the Chevy hubs and are not universal.
2) Be sure you have taken the metal clips off when you have the custom wheels put on.
We are having another set of wheels put on tomorrow. I expect it will still vibrate. I did learn there is a place in our town that does tire balancing with the tires/wheels on your vehicle. It's an old timer way that is suppose to be a little better because it balances them to your vehicle, but I haven't tried it yet. Wish us luck!