The SUMMIT replacement balancer has worked well for me.

Pioneer makes a decent balancer, and also for some of the harder to find stuff, dampner dudes can re do dampners. A word of caution though, we have had a couple dampner dudes dampners off by a couple degrees. No matter who's dampner you buy if you have the capability of verifying true TDC on your dampner I would recommend doing it. Last tip- white out-( the stuff for covering pen mistakes) works good for marking degree marks on dampners and shows up well with a timing light.

I had good luck with professional products dampers too.

Check the flexplate to see if the weight fell off of it. As far as the dampner goes, even if some of the rubber is missing but it has not turned it will still cause a vibration. 

On a somewhat related note...  On my daughter's 2000 Mustang V6, it developed a terrible vibration that felt just like a harmonic balancer went out.  Check the TDC marks and it all lined up.  Pulled the balancer to check for cracks or rotated outer ring or other damage.  Found absolutely nothing wrong with it.  Put it back on and tried plugs and wires, etc.

Well, it turned out to be the balancer after all.  Bought a new one and put it on and the thing ran as smooth as brand new.  That one still baffles me to this day.  And that was on the advice of a high school kid at OReilly's whose dad owned a mechanic shop and said they had seen the same thing happen before.

 

You only need one piston stop unless you plan on losing some or if you are getting multiple thread sizes. 

Thanks For all your help Al!
Good to see you and the Mrs again!

Are you sure the vibration is coming from the engine?  Is it there when you are holding whatever RPM at which you experience it driving while in neutral? 

Unbalance forces increase at a rate of the square of the speed.  So, vibration at 4200 will be four times worse than at 2100.  I can't tell from your post but does it start to feel bad at 2100 or is it worse at 2100 and better at 2500?

If it has a bad spot but gets better above, you are most likely dealing with a resonance issue.  Resonance occurs when you have a forcing function/frequency that excites a natural frequency of some structural component.  With a reciprocating engine, you will not be able to remove all forces such as engine firing and some slight unbalance so the best thing, when resonance is suspected, is to ensure that there are no structural natural frequencies that fall within the normal speed range of the engine.

One of the most likely culprits if it is indeed particularly bad at 2100 but better above is the exhaust system.  The pipes and mufflers are lightly damped in most cases which means that they can vibrate heavily with even slight provocation.  To test, drive some wooden wedges between the exhaust pipes from the engine back and see if the speed range changes or if it clears up.

Another possibility is that the engine and transmission assembly has a natural frequency on the mounts at 2100 RPM.  Loose or old mounts or combinations of urethane and rubber may create a natural frequency in this speed range or the mounts may not be providing proper damping.

The last thing I can think of for something that is very speed specific/sensitive may be engine misfire when unloaded due to a combination of mixture since you are still on the idle circuit (assuming carb) and advance.

But, before I ponder on other speed sensitive items, please confirm that whether it gets worse and worse or if it is only at 2100.  From what I have heard from others that have gotten the flywheel wrong (28 versus 50), the car shakes badly at idle and gets far worse as you go up in RPM.

But does it shake at all speeds?  Is it bad at idle?  Does it get far worse above 2100?

At resonance the vibration in a lightly damped system can be quite dramatic so don't discount it.  In troubleshooting you have to consider all possibilities and the possibilities that "can't be" are often exactly what's wrong.

Although we tested the vibration and determined that the driveline was the unlikely source of your problem, there still is a nagging feeling that the tranny is involved.  Maybe a failed pilot bushing or some problem with the tranny input shaft or a mis-aligned tranny adapter plate.  I have personal experience with your tranny when it was in PO's vehicle, and it most definitely was the source of PO's bad vibration.
On the other end, check fan blades, compressor, alternator and ps pump alignment.  Vibration does not feel ignition related.  Exhaust is a long shot but is a possibility.  Vibration does not feel like it is coming from the chassis.  That's all I got.  I'm out.
Al

All,
So since I am going to have to pull the tranny, may address the following:
Flywheel - can I go with a lighter unit? Maybe steel 20lb?
Clutch - Centerforce II?
Replace pilot bushing?
Throw out bearing
Replace tail shaft seal
Replace tail shaft bushing?
Anything else while I am in there?