Have you checked your Uni joints for tightness? Are the joints too hot to touch after you have driven for a little while?
You might want to check out if you have bent the shaft or lost one of the balancing weights.
Get it checked out by a "Good" drive shaft mob.
Another thing worth checking is if your wheels are round. They may balance up on the machine ok but if they're a bit square from pot holes then that will give you the sort of vibration your talking about.
And are your wheel nuts equally tight and are the rims square on the hubs?
And if theyre aftermarket rims do they fit snug on the hubs or are you relying on the wheel nuts to keep the wheels true?
You didn't mention if this has been a problem since you bought the vehicle or if it has just recently occurred.