because that slip and ujoint is a weak point. once you add hp ,in our drive train that is a place to start,, the pic is of my spare, just in case,, but i have found with my 599 hp our stock drive shaft is just fine, it just needs to be checked and maintained, in 6 1/2 years with the V8 i only replaced it once. , if you here a knocking or high pitch squeal,, (on hard take off) then check your drive shaft.
Note:because of the bearing it is hard for anyone to shake the shaft and find it bad. i had three mechanics say mine was fine,,including myself, then i took it out of the car and found it bad. jim