rear spring

Don't forget when the spring was changed, the resonant or natural frequency of the vehicle just changed too.  A small out of balance with the old spring which may not have been noticeable may now be coincidentally at the same frequency as the resonant freq. of the system.  This would cause an amplification of the vibe.

Thanks guys,
I did what Scott said and run 22 pounds of air in the tires and took it on freeway, guess what...it went away.
I'm going to get the tires re balanced this weekend and try it, if that isn't it it's to the shop.