i own 1994 chev blazer full size had on going problem for 4yrs.major vibration when reaching speeds of 50mph and up only when i reach that speed and let off the gas,and procede to excellerate thats when the vibrating begins. please help

Optional Information:
1994 Chevrolet fullsize blazer 350

Already Tried:
ujoints,driveshaft,tie rod ends balljoints,