VAN WANDERS AFTER NEW TIRES

Any input would be great on this problem. I had 2 new Firestone TransForce HT tires installed on the rear (fronts Bridgestone's and in good shape).
Before the new tires the van tracked like an arrow. NOW the rear is all over the road......feels like there is 20lbs of air in the tires....sloppy.

I also installed new rear shocks (Monroe Magnums) at the same time. I have a steering damper and Bilsteins up front both low miles.

For 10 years I've run 245's front and 265's rear with no problem whatsoever.

I did a search on the net and apparently this "wandering" is not unusual. It comes up in numerous blogs. I can't pin it down to only Firestones however.

It is bad enough that I wont drive it on the freeway. So far I have 800 miles on them and had them RE BALANCED AND SWAPPED SIDE TO SIDE WITHOUT ANY CHANGE IN WANDERING.

Like I said before the Firestones were installed the van ran straight down the road. Front tires wearing fine and no pulling at all.

Tire dealer is calling the "home" office for advice.

Anyone have a similar experience?

Thanks,
Steve