The two mounts are not the same. Would swapping them cause the engine to shake (not misfire) at 65-75mph?

I would not think so. One of the first things I would take a look at would be plug wires. I have seen them so bad on some vehicles that you could open the hood in the dark with the engine running and see the "leakage" sparks.
Jim
'85 300D
'95 E320
'97 CRV

Thank you for reply,

I do not believe it is an ignition problem: the vibration is present in a continuos wave; its amplitude varies with speed; it does not go away when coasting. The amplitude becomes first apparent at 2k RPM @ 50mph, then increases to a very noticeable shake, smoothing out after 80 mph, only to sharply increase again after 85mph.

The previous work included camshaft replacements, during which a timing chain was split & allowed to drop into the engine. Rotating the crankshaft during chain extraction attempts could have damaged the valves, however, the compression is OK & there's no increase in oil burn. Eventually the front of engine had to be opened & the timing chain was replaced. Transmission was replaced but not the torque converter. All three mounts & both flex couplings were replaced.