grinding under what conditions? My first guess would be the brakes if the
grinding is on flat ground while moving. (as opposed to suspension noise
going over a bump)
Best to jack up the vehicle and inspect the brakes with the back wheels off.
Might be just a pebble that got caught between the disc and back plate or
may be worn pads. Definitely worth checking.

Does the sound change when the E-Brake is gently applied? The rear brakes
are compound - disc for regular braking/abs and mini-drum for parking brake.

While you have it jacked up, grab the wheel at top and bottom and wiggle.
Repeat wiggling side to side. (use jack stands in case the jack slips)
Might be a cracked spindle bearing. there should be no wobble/play in the
bearing.

The back service brakes are single piston, sliding caliper. My '01Lw had a
bound up pad that caused uneven wear and poor braking. I used small
quantities of brake lube (heavy silicone grease) on the sliding surfaces
after thoroughly cleaning off any rust and ensuring that the pads moved
smoothly. Hasn't been a problem since.

Oppie