Your wheel bearings are shot, the hub nut (big 36mm hopefully with a cotter pin) is loose, or both. Jack up the wheel, release the e-brake, grab the wheel top and bottom, and wiggle. If the whole axle wiggles, including the cv joint cup, your wheel bearings are bad. If just the drive flange wiggles on the axle, your hub nut is loose. Remove cotter pin, put tranny in 1st, tighten until engine starts to turn, put e-brake on, tighten until e-brake slips, lower down, tighten really bloody tight (150ftlbs), keep tightening until cotter pin goes back in. This multi-step tightening process is to make sure the drive flange ends up centered on the shaft and bearings - if you just crank it down with weight on the wheel, I've had them end up crooked. Edit: Another thing I've seen is your caliper has come loose (bottom bolt backed out) and is rubbing on the wheel, but that usually causes loud clunks and occasional lockups, not grinding and occasional lockups.