Vibration at 50-60 mph ('93 3/4 ton C2500)

My '93 C2500 has a vibration that occurs starting at a little over 50 mph, and fading away as I approach 60 mph. It vibrates whether in o/d, drive or neutral. Any thoughts?

Have you checked to see if anything is loose, or if the tires are out of balance?

Please make sure you check U-Joints. My '99 Burb had this problem.

Thanks for the replies - the tires were balanced with no difference...I'll have to check out the u-joints