Have you checked the plugs, plug wires and caps, and distributor rotor and cap? Try removing the plug caps one at a time while the engine is running. If you remove a cap from a running cylinder the engine will vibrate worse and the revs will fall. If you remove a cap from a non-running cylinder it will make no difference. A 6 cylinder engine running on only 5 or fewer cylinders will vibrate quite badly, especially under load. If you find a bad cylinder, pull the plug and see if it's oil or carbon fouled. If it is, it could be caused by a bad valve guide seal letting excess oil into the combustion chamber. If you find coolant on the plug, you could have a blown head gasket.

Try the simple things first.

Just a thought