2002 Chevy S-10 2002 S-10 vibrates between 40 and 50 mph

2002 S10 4x4 pickup with 103225 miles vibrates between 40 and 50 mph. Above 50 and below 40 things seem to smooth out. I replaced all four tires. I had the front end checked and replaced upper and lower ball joints for both side as well as the idler arm. Truck still vibrates. I returned to tire dealer who rebalanced all 4 tires. It did not fix the vibration. Tire dealer then switch front tires to the back and the back to the front to try to determine wheather the problem was the front or back set of tires. I don't have the vibration in the steering wheel now but the truck still vibrates. The dealer put new tires on the rear. The new back tires didn't improve the vibration problem. Can you tell me what else I may need to have checked?