Vibration

Got a bad vibration at 70 mph that feels like its from the rear. Trying to track it down. Have a new drive shaft. 8.75 4.10 gear 742 that had the yoke changed.It was retorqued with the same parts nothing disassembled.I am afraid this may be the problem. I did find a cracked brake drum. Can this cause a bad vibration at speed?. Not braking just going down the road?. thanks.