I think your rear-end is alright. My full-size silverado has a limited-slip rear-end. I just jacked it up and checked it out. Yeah..even if you have the vehicle in park the tires will still turn (They should turn in opposite directions though). And you're driveshaft doesn't turn.
If you only have one wheel off the ground you won't be able to turn them while it's in gear. Only if both are off of the ground. SorryI didn't read the post right. I thought the driveshaft and the tires were both rotating while it was in gear.

 

the mustangs have a traction loc rear in nuetral both wheels should spin in the same direction an open rear is opposite...one wheel will spin forward the other in reverse... trust me my mustang is jacked up right now. anyway your issue is the brakes not the operation of the rear as stated above I'm sure your rear is fine.

 

you shouldn't have to put tons of muscle into it but it will have some resistance. I'd pull the wheels and drums and inspect all components like wheel cylinders = make sure the wheel cylinders aren't frooze... you should be able to move the shoes side to side and watch the piston move in the bore by pushing on the back side of one shoe then go to the frontside and push it back the other way

 

thanks, i'll try that.