Thumping Noise Only in motion
 

Hi there. I have a 98 Blazer 4x4. No weird noises when truck is idling.
When accerlerating, a slow rhythmic thumping begins and gets faster as I go faster and gets slower when I slow down. When I am going 35 or faster there is no noticable thumping, but there is a droning sound instead. The droning noise is quite loud when I am doing 65.
Any ideas?