While in drive and coming to a stop the car starts to vibrate or shake a bit due to low idle. when the car has the heater or air on the idle bumps up a bit and the problem is gone. can i raise the idle 200-300 rpm?

Define the issue a bit Further. does check engine light flash while stopped? check the basics is there air restriction and when where plugs last changed?

I had a 95 Volvo with similar behavior and I found out it was due to vacum leaks. I made my own automotive stethoscope to find the air vacum hizzing noise made by a broken vacum line or a air intake gasket or seal. You could by one of these stethoscope at any auto part stores

The cause is too low an idle and engine is starving for fuel so it shakes like that.The thumping sound you hear is most likely just the exhaust being too close to the floor amd bumping against it when in drive with brake on.You said yourself that it stops when you put it in nuetral,well the engine speed goes up when there's no stress on it.With some one holding the brake and it's sitting there thumping,set the idle speed up just enough to make the engine stop rocking that way.

I am going to assume the check engine light is not on.
I would check the throttle body to see if it needs to be cleaned, and check the idle air control valve to see if it is working. The computer controls idle speed by the amount of air coming into the throttle body.
The ac problem is probably something different such as low freon or a cooling fan problem.

It sounds like your a/c compressor is getting ready to lock up.

it could possibly be ur idle valve(air)....if there is a hissing sound when u cut the car off??? spray break cleaner toward the engine block and if the shaking stops its def ur idle valve

shaking while idling means that base of engine may need to be changed.

It seems that your engine is tuned to very low idle revs, hitting the low resonant frequency of the car at tickover. When in neutral or revving up (moving), the engine stops shaking.
You can have it tuned to be slightly higher so it won't idle so bad, but you'll lose a little bit of fuel efficiency in the bargain.
Alternatively, just shift to neutral - free and easy.

You are on the right track with the struts. With 212,000 miles, it is a great possibility that your shocks are worn out. With extra weight in the car, the shocks are suppose to keep the car from "bottoming out".

The creaking noises can also be caused by this and will be worse in cold weather. Does the creaking seem to go away a bit when you have been driving for a bit? When the gas in the chamber of the shocks warms up it expands and is able to hold the car up better.

Now, for the main problem. Usually vibration when braking is cause by warped or damaged rotors. The damaged rotors have probably caused the brakes to go bad as well. You can have a mechanic look at the rotors to see if they can be turned which will give them a flat surface. They may have to be replaced. Other causes could be the tie rod wearing out or wheel bearings wearing out. If the only time the car vibrates is when you brake, then more than likely it is your rotors. The ABS sensor is located very close to these components. It is likely that it is a faulty code and can be erased. The sensor may have been damage due to the excessive vibration. I recommend finding a mechanic very soon to look at your rotors.

If this happens when you are idle, and then you get hot air as you drive, most likely you have low coolant and it only cycles as you drive. Get approved dextrom gmc coolat and cap off your coolant it is most likely low.