Deffinately drive shaft related, I have had shafts balanced, but some shops don't have the proper equipment to balance a shaft at the type of speeds it will see under the vehicle, so hunt out a "good driveline shop" and pay the $$$ to get it balanced correctly. or just buy a new shaft from Inland Empire
http://www.iedls.com/page4.html
That link is to the new 2 piece shafts, completely new and balanced, spendy but perfect IMO. A little wobble in the shaft will create that speed sensative vibration so this is why simply balancing a almost 40 year old shaft may not "fix" it?!
BTW, did you rotate and rebalnce tires, just to eliminate that as a variable?? If so then the shaft is the issue, replace it and I'll bet the issue goes away!