these cars can make some very very odd noises... but, if'n it were me, i'd start with the u-joints, look carefully for any sloppy movement while torquing the wheel.  Next while its jacked up, push and pull on the tire with hands at the 12 o'clock and 6 o'clock position to see if there is any excessive movement in the bearings... otherwise, you might have something with the parking brake that may have come loose....rolling around and getting caught up in there..sometimes  it is hard to locate a loose beaing or u-joint while the half shaft is still connected.

I'm gonna go out on a limb and say you have a spindle bearing that took a doo-doo. They don't always just lock up. I drove the '74 up to the 5th Anny at the NCM, then up to West Virginny and back in the same trip with the rear bearing on the driver side howling. It didn't get really bad until we were almost home. Could also be a u-joint or park brake. U-joint noises can sometimes come & go, as could parking brake components that are floating around.

by most of the types of noise it sounds like a bearing but what about the differentials clutches, it would be easy to have the rear serviced with the proper lube if the 68s are the same.

Sounds like you have the same noise mine made before the bearings exploded..I thought it was just the parking brake..but it was also the inner bearing..not fun to have let go at 60 mph..I am doing a 73u rear suspension now if you look up my posts on it..I can go into heavy detail if need be.
Rich
yostusota2011-06-08 10:41:08

You may have a bearing out but I'm with rraider1, change the oil in the rear end and put back the proper oil and the proper additive.  It is worth a try and way less expensive.