vibration while on the gas?

[Terry]

I have an 86 ext cab. My truck has started to make some noise under the rear of the truck and it wobbles until its up to speed the it goes to a low hum. When I let off the gas it stops. I think that it might be the support bearing?

 

[Bob Ayers]

It could be a U-joint.

 

[ForOffRoadDriving]

get under there and check it out, feel it up and make sure its tight.