I own a 2001 z28 that has a dull but strong vibration that is noticed constantly,but is really noticed when I remove my foot from from the accelerator and the car slows from 65 mph to 60 mph.(AROUND 1700 RPM) I just recieved it back from the CHEVY dealership today for the 6th time trying to chase the problem down. And the vibration is still there. we have replaced:
BOTH UNIVERSAL JOINTS
TRANSMISSION MOUNT
ALL O2 SENSORS
CATALITIC CONVERTORS

ANY IDEAS??

Well, could it possibly be trans converter lockup trying to dis-engage? You mention that particular speed range, which is in lock-up range.

Also could be pinion angle.

i would look at the torque arm bushing, where it attaches to the trans, but with the big shotgun approach your dealer used, I'm kinda surprised not to see that on thier list!