96 GV Rear Wheel Bearing

Have a significant shimmy that starts about 40mph in the rear (based on seat wiggle and steady steering wheel) and lessens at higher speed. Had wheels balanced and rotated and tires checked for defects -- no change. Suspect rear wheel bearing and will know better tonight when I put the van up and see if I can move the wheels back and forth.

Looking for advice or technique in changing the hub assembly (bearings come as sealed unit with hub):
Should I do both sides?
Is the locking washer reusable?
What's going to bite me?

Thanks