rear end help
 

ok so when im driving down the road doing 70 or 75 and i come up on a vehicle and then pass it, i there is a really loud vibration coming from the rear end of my truck. this only happens if i really lay the hammer down and accelerate fast from 70 up to say 100. i have had this looked at and i cant figure out why it vibrates so bad.. any input or advice? thanks:tu: