Noise between 45 and 70 MPH?

1981 Olds Cutlass Supreme mileage: 95,848. I know that this is a hard one to answer without doing the road test yourself. I guess that I am just asking you to tell me of the most likely places to look. I am experiencing a thrumming sound which can start anywhere between 45 and 70 mph. I have only driven this car about a thousand miles since I bought it a month ago. I had to replace the upper control arm bushings on the left side and I bought four new tires and had the front-end realigned. I changed the differential gear lube (which showed signs of a very small amount of moisture present as foam) and the right rear axle bearing, which was really probably okay, but the roller retainer had a break in it. The races looked good. I repacked the front wheel bearings, taking out the outer ones and cleaning them thoroughly but just repacking the inners since I did not have the seals. The thrumming is inconsistently directional. By that I mean that one time the thrumming will stay or get worse if I make a steering correction to the right and quiet down completely and immediately if the correction is made to the left. A little later the correction to the right will be the one to quiet the thrumming down completely and immediately and the one to the left will establish the thrumming again. The universal joints seem good and the thrumming occurs even if you slip it into neutral and coast. What do you think? Where should I look next? Should I just keep driving it until is gets worse?