I have a 1991 Sierra with a 305. There is a vibaration during acceleration upto about 40-50 MPH. When you let off the gas the vibartion disappears until you give it more gas. At 60 it's just as smooth as anything. Any ideas?

I thought maybe wheel balance but I'm not sure.

 

ight be a ujoint.

 

Thanks. I had a sneaky suspicion that would be the case.