Tell them they need to verify that the rims are indeed round and have no runout, and that the tires have been mounted correctly so that they have exactly the same diameter all the way around. A piece of chalk run just barely touching the tire on the balancing machine will tell you. If the tire isn't perfectly round, the chalk will not mark it all the way around (a paint marker works well too). Takes a light touch, but if there is a bulge or out of round condition, the tire will vibrate at high speed.

A bad belt, or a hard spot in the tire, where it flexes less, will also do this.

You should also attempt to determine where the vibration is (right/left, front/rear), and have them move the tires around to see if the vibration follows the tire. If so, I'd insist on another tire or pair of tires, they are either defective or improperly balanced. I suspect you must have a dynamic balance done on a good machine, the simple more or less static balance won't work.

If they are not V rated tires, swap them for some that are.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!