Have a 05 V8 4WD with 245k
I’m an environmental consultant, I beat my truck to death, constant rough gravel roads littered with huge pot holes. The ride is spine breaking jarring....

I’m running 5100s, and Michelin’s.
Front I’ve replaced LCA/UCA bushings and ball joints, sway bar, tie rods etc.
The front end is tight as it should be.

When I hit bumps the front takes them well, but the rear just hops and shakes all over the place. I’m thinking the bushing in the rear suspension arms are shot, but they look okay upon inspection.

Also thinking about replacing the body mounts, they have been beat, but I can’t find anything about those who have done this.

Just trying to figure out where to begin...

If the rear is hopping, then make sure the rear shocks aren't shot.

Definitely check out the shocks to make sure they aren't leaking. Also you may be on to something with the arms! All four of my rear arm's bushings were shot or on their way out; the ride was not bouncy because I had new shocks, but boy was the rear end harsh going over bumps. New arms with fresh rubber made a HUGE difference.

You're on the right track with the bushings. With your miles and type of driving conditions, even though they look ok, they will be worn out.

New body mounts will definitely help reduce the harshness and vibration that's transmitted to the body too. What tire pressure are you running?

Since your doing plenty of offroad work, I also recommend upgrading tthe shocks and coils to a properly matched set.

New body mount bushings are expensive. Around $500.

4th Gen Poly-Urethane Body Mounts

Sounds like shocks to me: I'd suggest, based on your driving, going for a shock that's designed to absorb the heat from the higher speed driving on gravel.

A reservoir shock would probably be best albeit $$$

Choosing the Right Shock Absorbers for Your Truck | 4WD.com

I was bottoming out with stock suspension in the rear. I added a 2.5 inch spacer in the rear and now it rides much better. The long term goal is proper suspension but the rear coil lift helped immensely.

Great suggestion, as well as all others above.

I was bottoming out my rear suspension constantly... tapping the axle into the bumpstops. I didn't feel the need for extra ride height for mall crawling that I do, so I put on a set of superbumps.

In the OP's case, the extra ride height would be great, a resevoir shock that can take the heat generated from constant cycling, and a set of superbumps for the occasion where the axle needs a big deflection upwards (and gets gently slowed by the superbumps, not rammed into the stock rubbers)... should dramatically improve performance.

Assuming that the bushings aren't shot and the arms are the biggest issue anyway.

If you’re on your original springs, I would start there. Could be a relatively easy & inexpensive fix.