My 2000 ES300 had started experiencing a few issues: I've addressed most mechanical issues:

New struts, front and rear, new mounts, replaced some leaky gaskets, lines for power steering, all hoses and belts, wiring harness and coils.

Problem is, though, now the ride is extremely stiff and I'm getting a rumbling sound from the rear end. Is there anything that can be done to reduce the stiffness? Did my mechanic do something wrong? What might be causing the low, droning rumbling coming from the rear?

I've loved this car for a long time, but now at 129k in miles, so I want to do things right and keep her around for a while.

 

The stiff ride depends on the struts that were used. Some aftermarket struts are harsher than stock. The rumble could be a side effect of the new suspension. Best would be to ask your mechanic to check it out.