Just so everyone knows, my ford explorer had the vibration, they replaced the bolts, gears in the rear end, driveshaft, new shims for driveshaft, and the new transmission and it was ok for a while, now the vibration is back at around 40 MPH especially when the car stands for couple of days it gets worse.
This means there is something else there that causes it, maybe its the transfer case, I am at loss now. Someone must know what that is and admit to it, but have not found anyone.