1990 C3500 crew cab vibrations

I just got a 1990 C/K 3500 Crew Cab truck, and had to drive it home about 600 miles. Made it OK, good truck. However, I noticed a vibration in the chassis/drivetrain that only shows up at speed (60 mph and up). I put the engine into neutral to idle, and vibe was still there. This vibe can be felt in the floor, steering wheel etc... really a whole truck experience. It isn't anything that feels like it's going to fall apart, just something out of balance maybe?? Puts your fingers to sleep on the wheel! I noticed this truck has one of those long, multi peice supported drive shafts. Can anyone share if this sounds like this could be an unbalanced drive shaft maybe?

Oh yeah, this vibe didn't start until about 100 miles into the trip, so maybe a weight fell off something????

Never dealt with these HD trucks before....

When you said the vibration can be felt when you placed the trans in neutral, this indicates a vehicle speed related vibration.
Diagnosis would look at rotating componants such as tire/wheel assemblies and their related components and propshaft and its components.
For proper diagnosis, driving the vehicle with a reed tach or an electronic vibration analyzer would give you an indication as to which rotating component you would look at.
12hz..tire or related component out of round or imbalance
24hz. tire or related component bent
36-50hz propshaft out of round or imbalance
70-100hz u-joints or working angles