Harmonic Balancer? Or a sensor

You know...a harmonic balancer actually makes a lot of sense. I'll check it out.

Check all the mounts. I'd say start at the front trans. Sure you've had it replaced, but that doesn't mean it didn't rip in half.

Been there, done that in one stoplight.

Are all engine mounts generally this weak, or is it just a GM thing? And are there stronger aftermarket parts available? (just asking in case a mount is actually broken, but I don't know yet)

Go look....

Are your motor mounts 19 years old? Are your mounts dry rotted or degraded? Do your mounts have over 100,000 miles on them?

Wouldn't call mounts typically weak, cheap or crappy. Toss on a pair of $500 sneakers and walk 100,000 miles....let'* see how they look when your done.

I see your point, and some of them are probably OEM and it makes sense they will break, but when you say "Sure you've had it replaced, but that doesn't mean it didn't rip in half," makes it seem like they are VERY easily broken.

Approximately where are the other 2 besides the front trans and torque mount? Would they be obvious if I could get under the car?

You got your dog bone near the passenger side axle. The other one is on the backside of the trans right in front of the steering rack.

When was the last time a tune up was done?

There hasn't really been any specific time I did a tune up but over the two years I've owned this car I've replaced the spark plugs, plug wires, fuel filter, air filter, supercharger coupler, UIM/LIM, valve cover gaskets, injector o-rings, IAC, belts, a pulley, water pump, oil pump, some vacuum lines, and of course the big one: new crankshaft and bearings, oh and oil changes and coolant changes. Ugh...it hasn't been cheap. I've also checked most of the sensors with a multi meter, and electrically, all are in working order. So realistically, I'm left with either the harmonic balancer, engine mounts, or the transmission.