I have a 98 Silverado 5.7 Vortec Z71 all stock. I get this vibration in my truck when it reaches about 50 mph. I've also noticed that when using cruise control and I happen to be going down a hill the vibration goes away then begins again once the cruise control takes over. When I get on the gas excessively from a dead stop, the vibration comes right away but then disappears until 50 mph. I've had the U-joints looked at and they are OK. A front end alignment was done about 3 weeks ago. Any ideas/suggestions??

 

A guess... motor or tranny mounts?

 

My guess is the transmission. I hope I'm wrong. Good luck.

 

driveshaft could be out of balance

 

Carrier bearing.