Slight hum, wobble or shimmy from right rear...at least I think

[D-Day]

1973 Chevrolet crew cab, 2wd, 400 V8 Blazer engine, C20.

Now, I've had this humming problem for quite some time. I had the rear end gear changed to a 3:73 in November 2004, and when the guy put the case back on he only used silicone instead of a seal or some dumb crap. Bottom line, the rear end leaks a bit, and eventually I gotta refill the thing quite often. Now, the reason I mention this: I was under the impression that the rear end would hum at higher speeds ('round 62+ mph) because it didn't have enough lube. Then it seems like it hums at highway speeds even if I pack it full of lube right before a trip.

I have a TH350 trans, and it's running about 2800 RPM's at 65mph. This noise is annoying on long highway trips, though I can get used to it and drown it out. Now, the two trips I had to deal with this on to St. Louis (460 mile plus trip), I had the old tires that came with it, old Cooper's that were probably ten years old by the time this trip was taken. Yeah, I know, that can be a cause. However...

My right rear tire busted a belt and started to wobble bad at certain speeds here around local summer running; I was going to change the back tires and wheels, but the right rear blew before I did. I thought, well, the belt was busted and caused the trouble. I'll throw on a couple mud boggers I have to hold me over and it'll be good to go.

Thing of it is...the hum/shimmy remained. Now, this isn't a big wobble or noise. I'm just extremely perceptive of sounds or something going wrong in my truck; much like a race car driver is always listening for something going wrong. Perhaps I'm just paranoid; it is a 33 year old truck. But damnit, if there's something hinky going on, I want to know exactly what it is, how serious, and if I should fix it.

For example, the right rear will still feel funny just little at 20-25mph, then disappear at speeds higher, though it won't wobble, it hums at highway speeds. My passengers never notice it; even if I turn off the radio and make them feel or listen for it, and maintain the speed it does it at. Again, paranoia? I've kept attributing the humming noise to the rear end fluid constantly going down to the leak; however, the old tire had a busted belt, and now I have old mud boggers on there to hold me over for a while. You know what a mess this is to diagnose?

Now I'm wondering if it could be the transmission, because I had a passenger feel some of that same vibration/humming from the hump, something I haven't and don't. I'm looking to upgrade my trans to 700R4 soon, so we'll see if the reduction in RPM's changes anything. I know a decent set of tires will help. Until I get new tires, I'm gonna have the rear set checked/rebalanced and see if that helps.

Another thing I thought of was the driveshaft, or perhaps the U-Joints back there. Someone mentioned perhaps a bent axle...but if it was bent, wouldn't it wobble, hum or shimmy at nearly all speeds? This problem seems to be intermittent and hard to trace down.

On top of the transmission, I'm looking to have the rear end seal replaced so the damn fluid doesn't leak out when sitting. The guy I'm having replace the rear end seal has worked at a GM dealership for over 20 years, so I'll have to ask him a bunch of questions. I don't expect an answer here, but to see if anyone else has had the same problems, what their situation was, and what they did to fix it.