time for new shocks?

After 89k miles your OE shocks are definitely toast. I put the Reflexes on at ~60k and can recomend them highly. Felt like a new truck...

-Fred W

It may feel like a new truck, but that's not likely the cause of a vibration. The shocks stop the springs from rebounding endlessly after hitting a bump.

A vibration has to do with something spinning out of balance. I'd be looking very closely at the tires. Certainly not unheard of for a fairly new tire to experience a cord seperation. You might try to find a shop that can spin them up individually on the car to see if you can isolate one causing a problem. There are also driveshaft issues to be considered, engine vibration at that particular rpm (although you should be able to replicate the problem in neutral) from an out of balance fan (I've seen them lose a blade). Many possibilities, but look for things spinning, not bouncng. Sometimes a shock change will reduce or remove a vibration from an out of balance wheel because it reduces the oscillation, but all you're doing is covering over the root cause.

You may well be due for shocks at that mileage, but again, they are not the cause of your sagging rear. Schocks don't carry weight, the springs do. If the back is sagging more than it did it's because the rear springs are weak. Im not a great fan of compensating for weak springs with an air shock or spring assist shock if the vehicle wasn't originally equiped that way. We used to see kids in the 70's jack up the back of their cars with air shocks, and would also see a lot of broken shock mounts on the frame or the axle. If the shock mounts were not built to carry load, just to absorb shock, you may find yourself in that same boat.