A little bit of it feels like its in the steering wheel. I think that is coming from the loose play in the column but I could be wrong. The majority of it feels like its coming from the wheels or under the bed. I haven't tried getting the drive shaft balanced but may give that a shot now that you mention it.
As for the info on the truck its a 72 LWB, 250/350. It does have the 2 piece drive shaft. Its stock height as best I can tell unless the previous owner lowered it but it doesn't appear to have been. It does sit a little low in my opinion but I think that's from saggy springs (yes they are on the replacement list
) But if you have other ideas of what I should check I'll happily check to see if that gets the shake completely out of the truck.