Need help, oscilating vibration at 50-55 mph.

[mr27t]

I drive 80 miles round trip to work and on the way, I noticed an oscilating vibration at about 50-55 mph. The vibration comes and goes at a rate of about 1-2 oscilations per second. I don't feel it in the steering but I do feel it in the seat of my pants and on the floorboards. I'm thinking that it could be a possible drive shaft pinion angle problem. I have a Jaguar differential. I don't think that it is a drive line ballance problem but I could be wrong on that.
Anyone have any ideas to whare I should be looking to resolve this problem. Any assistance would be greatly appreaciated.
Thanks

 

[Youngster]

What angle is your rearend set at? I have heard tires that are out of round will feel like that too.

Ron

 

[TheAntiBoyd]

did someone sell you flintstone tires instead of firestone?