[mr0077]

Jim, Hi, I'm a former JH owner, number 14710...absolutely GREAT car, wish I still had it (except when I got caught in a sudden downpour...took ten minutes to put up the top in a nine minute deluge...2 inches of water in the floorboards)!
Anyway, if the vibration stops when you depress the clutch, it seems like it has to be in the parts that rotate with the motor. If so, you could go make the "hop" occur, depress the clutch to make it stop, then repeatedly rev the motor to see if it followed the motor RPM rather than the "driveline/road speed" RPM. If it doesn't follow the motor RPM, then I'd have to believe it is either something in the bell housing (bent input shaft, but how the heck or, like the last post, the motor mounts, particularly the left one. If the left mount is separated, it could allow the motor to go through a series of lift/fall cycles during acceleration...that motion would stop during deceleration or when the clutch is pushed in, FWIW.
Let us know what you find.