Ok, lets start from the beginning... Your stall speed is around 2000ish depending on how much torque your motor puts out...
You were allowing your motor to make enough torque to make the converter to sit at 2000rpms until the drivetrain speed catches up with the engine speed.
Torque converters can lock up... Meaning the engine speed always is a linear function of the drivetrain speed... X rpms always = Y mph when the converter is locked... Its hard to explain... Basically when the converter is unlocked, Its like you're slipping the clutch in a manual transmission (But no wearing out parts)... You can be at whatever RPM you want at that speed, but depending on how you modulate the pedals, you put out a different amount of power to the ground. Lockup would be the clutch completely applied, holding the engine and transmission spinning togeather. Your car is only programmed to lock up in 3rd and 4th under low throttle.
Torque converter shudder is a condition our vehicles experience when the friction of the torque converter clutch (to lock the converter) and transmission fluid breaks down. As the fluid gets hotter, it doesn't hold as much friction betwen the surfaces. When a torque converter attempts to lock up, and the friction is not high enough between the clutch and the case, it slams between locked/unlocked as it tries to hold togeather (wearing out the clutch even more)
Ok, I hope that helped make a little sense... Let me go through your scenerio...
Accelerating lightly 1st goes by fine, then 2nd to 3rd the revs drop to like 1100 from just under 2000 then immediately to 4th (this is around 38 to 40 mph). The car starts shuddering unless I push the pedal halfway to the floor(or hit the OD button) to bring it back into third. Almost like its skipping a gear but I can feel it shifting.
In 1st, and 2nd, your torque converter doesn't lock. That's why it doesn't shudder... After you shift into 3rd, your torque converter locks up (1100rpms corresponds to the speed your drivetrain is actually moving if you had a manual transmission). When you shift into 4th, your engine is still trying to put out roughly the same amount of torque as it was before, but due to the gearing difference of overdrive, the wheels are harder to turn. The torque converter clutch however isn't as difficult... So what happens, is they do battle until you tell one of them to knock it off... When you give the car more throttle, it's really not shifting into 3rd... its sending a signal to the transmission to unlock the converter. The symptoms stop, and the car accelerates to a speed where the converter can hold the applied pressure, the converter locks up, and everything is happy until next time.
Next time it does the shuddering, tap the brakes... The BOO switch sends a signal to unlock the torque converter... That way you can accelerate to a good speed without having to add more torque to the equation, or force the car to downshift.
Go smell your trans fluid... See what it smells like... Also, I'd check your throttle position sensor. It could be out of calibration, and reading lower throttle input than it's really getting, causing you to shift too early. If your coolant isn't **That** bad, a change couldn't hurt... Note, I said change, not flush... Just let it gently drain out, and put more in. A cooler helps a lot. My dad's car has BAD shudder, but in the winter, it doesn't show up for probably the first 15-20 minutes of city driving.