Where are the vibrations being felt, i.e., through the seats, the steering wheel, etc.?  The area the vibrations are coming from will help us help you to locate the problem.

Steering wheel vibrations usually means the tires i.e., balance, separation, out of round
Vibrations through the seats usually means pinion angles or u-joint wear.

Does the vibrations continue above 45mph, or do they go away?