When I make a sharp left turn I hear a thunk in the rear axel - tire area and today when I backed up out of a parking stall it made an awful grounding sound from the rear axel (tire area)

Could be universal joints if rear wheel drive , you can physically see by checking the drive shaft at each end. If its that bad it will fall. Also could be tires out of balance

Check rear brake, axel and bearing, the problem is purely mechanic on rear back wheel. You must put your car on a bridge to fix it.

The possibilities are depending on whether you hear the sound in the front or back of the car: You have a broken brake pad in the front or a broken brake shoe in the back; there is a bad wheel bearing that needs to be replaced, there is a flat spot on one of your tires.

Does the mustang have independent (IRS) axel or the solid 8.8 rear axel?

Outside tire turn's faster then the inside one. it is not a solid axle, might want to change rear end fluid and add some slip addative, go to a parking lot and go around sharp 15 time's one way and 15 time the other way, some time's that will loosen the clutch in the rear end, if that don't work you will have to take it in for repairs, the rear end is not turning freely from the other axle.

i would put money on tht being cv axel shafts

Check the tire on the front of the car for a broken belt in tire. Look on the back side for a blug,swollen area on side wall of tire. Raise the front of car and block the rear wheels, spin the tire by hand and look on both sides as tire is rotating for anything unusal. Hope this will help you.

Sounds like your half shafts are going bad. Does it make this noise when turning left or right? If it does your half shafts have gone bad.

Stationary lock to lock turning, will not damage the CV joints, they are designed to turn in this manner, to test a CV joint to see if it has damage, go to an empty parking lot, stop the car, turn the steering wheel full left, accelerate kinda hard, do you hear a clicking or clunking noise? repeat in reverse and on the right hand side. Any noises and your CV joints may be worn, also inspect the CV boots for tears and leakage by turning full left and looking at the boot, then full right. Hope this helps, let me know

just a thought but is it a new drum ? it could be defective (out of round) or are the shoes seated on the backing plate right or the emergence spreader bar in right way causing shoe to bindif not or shoes are on wrong if it has a primary and secondary shoes .And check to make sure lug nuts are secure and that the rim is not shifting on braking .that ii have seen lots of times