Mack granite rear end shimmies/vibrates when driving

If it started with the new tires, give it a minute. I've had new drives be all over the place until they break in.

 

Drive line angle? Driveshaft support bearing or insulator? Bent hub, bent rim. Drive axle alignments are off.
I would start by inspecting driveshaft carrier.
Than jack up drive axle in the air, release brakes and check for out of round wheels and hubs.

 

Also are you sure it’s rear end that vibrates? I had 2 Michelin steers that were old and causing steering wheel to shake, had some uneven wear and a started to dry rot. I replaced them with 2 new ironman steers( waste of $1200) Same shaking/vibration on steering wheel. Did alignment,(nothing changed). Than steers started to wear funny, probably because it’s cheap china.
So I replaced with Bridgestones and put centramatics and all the shaking and vibration went away. I was amazed how much of a smoother ride I got from decent new steers.