Vibration in the seat

Hello everyone thanks for all the help.
Know i have another problem, i just had my new leaf spring put on at toyota
i had the problem with the viberation before my springs were installed. i had to get new tires
and the problem is still there i changed my front wheels to the back and back to front and still have the viberation. the breaks front and back are fine, the steering wheel viberation is not as bad as the rear. so what can be causing the problem?

 

Is it only noticeable when accelerating or constant?

 

yes

 

both

 

I would check your u-joints and make sure they aren't going bad. Just try to rotate the drive shaft and if you feel any play that will be your problem. I just had to go through this about 2 months ago