I have a similar issue. It gets real noticeable around 70mph. It's like my bass is turned way up sort of a fine/buzzing vibration. My drive shaft and U-joints seem quite solid. There is a very slight amount of up and down play in the front of the shaft near the tans.
I tested and it doesn't have any leaks. Does anyone have a photo of their heater lines and where they connect? When I ran water through the pipes it came out this hole here:
What kind of lines do I need to get this all connected and working again?
The previous owner did replace the engine. My coolant level is fine so it's not draining. It looks like they did loop the hose like TheDurk mentioned. Is there a way to tell if it was leaking coolant?