Does your Duster "float" at highway speed?

So much can affect all this INCLUDING REAR SUSPENSION

Decent springs
Decent shocks
Decent tires, not just tread, but tires that will HANDLE Lots of cheap tires don't
Proper alignment. Don't just go changing alignment unless you have reason. The infamous "skosh" chart is a good start
Loose bushings, ANYTHING hardware loose can cause this