I know you stated that you had checked the engine mounts and they were ok, but what about the tranny mounts.
One more suggestion...
Since your problems appear to be happening when the driveline is heavily loaded, I would stay in the driveline.
I agree that it might be a clutch problem with the slight chance of it being the tranny mounts. You might have to get the flywheel (faceplate) inspected for "checking". It would stand to reason if the only time it does this is unextreme loads (heavy acceleration and heavy engine braking) and not under normal use.
