Do new tires effect body roll?

[Jeph]

I recently had new tires put on my car -just the rears. They are the same type as up front and were installed by the gurus at RUF Auto Centre.

BFGoodrich g-Force T/A KDW 2

The front tires have about 50-60% left on them. Everything on the car seems to be balanced and smooth.

However, my car at highway+ speed does not feel as surefooted and tight as it did before the new tires were installed. Even at lower speeds (~40mph), some quick zigzagging seems to indicate what feels like body roll. The same action before the new tires was tight and planted as could be.

Is this normal for new tires? Is this because I have new tires on the rear only? Could it be they are over-inflated (haven't checked the PSI yet)? Do they just need to be broken in? Should I switch out the fronts as well?

What do you guys think?