I have had this problem since I bought the car. At or around 80 MPH my car vibrates pretty bad, first I though it was wheels off balance, the the axles, then the drive shaft, over the years everything in the drive line has been replaced even the rear end. I put a different drive shaft in it, but it could be off balance.??

My question is, could the strut mounts in the front of the car be the cause for this? A friend suggested that, said he just replaced his and sails smooth now, but wondering if anyone ever heard of these mounts causing this becauase of the bearing in it? thanks in advance.

Yup could be possible cause. if the bearing is wore out it could be slopping all around.