Time for the radiator hose clamp on the rear half of the driveshaft...
If there are any balance weights on the shaft now, place the screw clamp head opposite the weight position. Tighten the clamp and retest.
As you test, move the clamp head around at 45 degree increments....you will find the sweet spot...
All shafts and rear axles have a certain amount of vibration...that's why the center carrier has a rubber damper in its design.
If you are still concerned that the vibration is too bad it may be time for a totally different driveshaft...but don't be surprised if the new one vibrates just as bad.
BTW...some large brake drums do have balance weights welded on them...maybe one has been lost...look for areas on the outer surface where a weight may have been welded on at some time.
Re: Heater core tubes...not the first time I have heard of this...are you sure it was the correct heater core? The holes thru the outer case are usually much larger than the core tubes...pics please...
Try the hose clamp trick...trust me...it works...
