One way to discern engine & driveline from suspension is to put in N and coast w/ engine idling. If vib's go away, it was surely from the driveline. If not, it might still be from an imbalanced driveshaft since that is still spinning, though less likely from U-joints since they are affected by torque too.

Perhaps not directly related, but I had similar in my 69 Dodge Dart long ago. It would start a humming vibration right at ~60 mph and go away above that. I found the rear U-joint (cross type) was totally rusted w/ the needle bearings missing so it just wobbled around. Replaced it and no more humming, plus no more clunk when I shifted between D and R. Must have been driving that way for years since I long wondered what that clunk was.

__________________
1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans