engine vibration

I have a 1989 Mitsubishi pickup.The engine is a 2.6l 4cyl 5 speed. I am having a vibration from my engine . It seems to be running fine. Its not missing and i checked the timing and it was ok? I have also inspected the motor mounts and they seem to be solid. It almost seems as if something could be off balance. Does anyone know if something internal could cause this. Its not in the drive shaft because it does this in neutral. Does anyone have any ideas. Cause Im out. I am thinking of ordering a remanufactured engine but I really want to fix this one. Any help would be Great! Thanks Tony

Its not in the drive shaft because it does this in neutral.

The driveshaft is rotating whenever the vehicle is in motion regardless of gear position. Is the vibration present when the vehicle is not moving? If it is than you can rule out the drive shaft. Does the vibration occur at a specific M.P.H. or R.P.M.?

Check every part with a pulley ( A/C, Alternator etc.) for unusual noise and secure mounting. Observe all pulleys with the engine running. A bent pulley will wobble and can cause vibration particularly the crankshaft pulley/harmonic balancer.

The 2.6L is equipped with internal balance shafts which are designed to reduce engine vibration inherent to all 4 cylinder internal combustion engines. Are you hearing any unusual noises coming from the timing chain cover? If you have an oil pressure gauge does pressure remain constant or does it jump around (the balance shaft chain drives the oil pump as well)? How many miles on engine? Has the timing chain ever been replaced and if so was the balance shaft chain and guides replaced as well?

I have eliminated the drive shaft because it does this when it is not moving. Sorry I didnt clarify that. I have also eliminated the cooling fan pulley , a/c , and water pump by removing the belts and starting it. It has 200,000 miles on it . I have only put 100,000 of them. But i have not changed the timing chain and hear no strange noises from there. The engine actually sounds very good until you get in the cab. Then you can hear and feel the floor , roof , steering wheel , and pedals vibrating. It does this at any rpm, higher or lower rpm just changes the pitch of the vibration. Also I dont have an oil pressure gauge but my oil light has not come on . I have also checked compression on each cylinder and read 150 on each . My Chiltons says this is about right . Well I am still stumped. Thanks for you ideas Delta Dart if you have any more advice let me know. If anyone else has an idea "be my guest"!!

Have you ruled out clutch components, flywheel or transmission mount/ attaching points? If the vibration is that bad in the cab maybe you are sitting over the problem. Any recent transmission/clutch work or problems? Any unusual noises from the starter?

Have you checked the balance belt. There is a smaller belt behind the timing belt that is called the balance belt. Mine broke several years back and it had the same vibration you are describing.