Brake squeal as we know it is caused by vibration of pads against a rotating rotor. Brakes do not squeal in this manner when stopped. I think you are mis diagnosing. Is the squeal continuous? If so why not walk around the car a bit, open the hood, look underneath and try to pinpoint the source. Have you inspected the vacume lines to the power brake unit? Is the brake fluid reservoir level dropping?

A shuddering at any speed can mean a warped or heat damaged rotor(s) on any wheel. A brake job or even a "full brake job" does not mean new rotors were installed or that they were even checked. A bad internal steel band or tread on a tire can give similar symptoms.
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David Hunter

I am having a hard time determining what could be causing brakes to squeal when not in motion, seriously, there is no driven pump of any kind associated with the braking system. All I can think of is something wrong internally with the ABS unit that would emit a sound akin to squealing, but I have never heard of the symptom and I have been servicing Volvos for 30 years.

However, a simple check of your fan belts may be in order. With the engine OFF, there should be no more than about 1 inch of movement when a belt is pressed. You should also not be able to turn the belt axially more than 180 degrees. Alternatively, with the engine running, you should not see any wobble in the moving belts.

If you can pinpoint a particular event associated with the squeal it might help diagnose. For instance, if the car is not moving but you are turning the steering wheel when it squeals, that would point to a loose pwr. steering belt, low P.S. fluid or a failing P.S. pump.

brake squeal on your 940 can virtually eliminated forever by the simple chamfering of your brake pads BEFORE you install them.

Jack it up one corner at a time and see how each wheel turns. Can you localize the noise to front or rear? When you mention the noise being worse in reverse it makes me suspect the parking brake assembly, which is notorious for seizing up, particularly on cars that have been through salty winter roads.

Pull each caliper and inspect the condition of the rotor on both sides. Check that the pads aren't binding up (I've had that happen several times on the rear brakes), make sure the pins are clean and have a thin smear of antiseize on them in the rear. On the front, make sure the pins on the floating front calipers aren't seized in their bores. Make sure the boots are in good shape. Use Volvo shims, particularly for the rear brakes. Make sure the pistons retract well in each caliper by pushing them back gently with a c-clamp. Looking at the parking brake assembly involves pulling the rear rotors-- it's best to have new parking brake hardware on hand if you are going to take it apart-- I've busted the parking brake shoe retainer clips getting rotors off.

For the shudder, also check the radius rod bushings in the front end. Re-inspect the outer and inner tie rods. Check the ball joints for excess radial play.

In the worst case scenario, take the car to a real alignment specialist (not Sears or some chain store) and have an alignment specialist give the car a once-over. He will find your problem.

Good luck!