Pulsating vibration while cruising??

Put jack stands under the rear axle and run the car. If it stops the problem is in the front end somewhere if not and it vibrates in park like Rusty said its up front somewhere. If it’s ok in park run it up to 2000 rpm then slip it into neutral and see if it goes away. If not take off the rear wheels and try again. Next remove the drums and try again. (Be conscious about avoiding the brake pedal) I just turned the ignition off and it stopped turning. See if you get any differences without those items. While it’s up in the air make double sure there is lube oil in the differential. Ya never know....