Did this vibration start a little and has got progressively worse? I would go back and check the basic things, lug bolt torque, maybe a thrown wheel weight, other repairs. Do you know if the tires are rolling the same direction as before the wheel balance? Some times this can affect a tire that sits in one position for along time, then starts rotating the other direction. Back in the 70's I had a Hunter on the car wheel spin balancer. If you know of someone who has one that could save a lot of time tracking down this problem and have a very accurate wheel balance because it spins all the rotating parts, drums etc. There are probably not many of these spin balancers around now. Should have kept mine. Hope this helps.   Part of the problem,  Cav Man

Is the driveshaft yoke tight in the rear of the transmission?  Either a worn yoke or rear transmission bushing can cause a vibration.  Rebalancing the tires using a Hunter spin balancer is also a good suggestion and worth pursuing. 

I chased a similar high speed vibration in my 55 Tbird for a long time.  Could never find a shop with an old school spin balance machine. I was about to give up and accept it when I bought new wheels.  I was running the original steel wheels.  I replaced them with modern aluminum wheels and remounted my old tires.   To my surprise, the vibration immediately went away.  My steel wheels looked to be OK and were balanced off the car.   I have no idea why my steel wheels did this.  All I know is the vibration is gone.

Good points. I'll get the wheels rotated and report back results. 

Not sure on a local shop with a 'road force' balancer. Someone on another board mentioned that as well. Will do some digging.

Thanks again for all the pointers.

WOW! Something is severely askew there. Since you mentioned driveshaft balance is an unknown I think I'd start there by taking it to a driveline shop and have them check it. I'm betting it is out of balance to the point that it beat up your new tailshaft bushing, hence the new leak. New u-joints would also be in order.

When that was happening; did you happen to reach in the car and put it in neutral. If you did, did the clicking slow to a stop?
I know this would a lot of extra work but I would have removed the yoke from the drive shaft and slipped in the trans and tried it again.

I don't know what I'm talking about but...

One time when I was having new tires installed on my truck, the guy doing the work got me to come look at something.  On the balancing machine, the wheel appeared to be wobbling.  What it was is the bead was, for lack of a better word, wavy.  I would guess that it was at least 1/16".  These wheels had been used for the previous 10 or 15 years and were never a problem in practice so I told him to continue and all was fine.  Whether that was a smart decision, I don't know.

RE the brake drum,that outer edge is not the most critical portion so I would imagine that it does not need to be exactly concentric.  The only machined portions are usually the the braking surface and axle mating portion.  Those should be flat and concentric

With the tire/wheel removed does the drum center fit tight on the axle hub purchase? To me thats the most important thing.
Also I have a parts store that still turns drums. I you have one near have the put the machine an look at it with him spinning it.
Next is applying the email brake when the drums are hot will distort them. A test is to slowly apply the emergency brake while coasting in neutral at about 35 mph and see what it feels like. Should be pretty smooth.