RE: Vibration while driving approx 40 mph
Are you sure the vibration is in the rear? I had a similiar problem that I noticed as occassional steering wheel shimmy. It was caused by a worn upper ball joint.
Even if the tires are balanced you still might get vibration or shimmy from a tire that has a bad belt. When I was troubleshooting the bad ball joint I had the shop I bought my tires from check for this during a tire rotation.They were actually the ones that told me exactly which ball joint was bad all for no charge.
Another problem could be if you have a 2 piece driveshaft. Awhile back I was reading up on a lot of problems associated with them. Might be worth the research.
KC '99 Silverado 1500 Z71+