I had a similar experience with my Windstar - lots of pad material left, but still grinding after new pads & shoes. Turns out the rotors were bad. :( Btw, I just replaced the rotors without finding out if a simple turning would have eliminated the grinding.
engine problems not trans.
brakes or rear axle problems.
They will make a grinding noise that's how you can tell.
Worn out brakes.
The brake pads are worn out and need to be replaced. The grinding sound means your brakes are about to stop working.
You should take it back to the shop that replaced them and ask. It's possible that something is hung up somewhere that's causing the noise.
Brakes
I would suspect your brake pads are worn out. The grinding you are hearing is the destruction of your rotors taking place. It is also possible that you may have a defective wheel bearing. Inspect your brake system.
Your pads or discs are worn. The brakes will fail soon if not replaced.
your brakes are about to give out... you should go to wal-mart and have them replace them
Does it squeek? Or is it grinding? Possible sand/ gravel trapped in your brakes. Or you need some new pads. If it's during winter and you break hard and start sliding.. then it would be your ABS kicking in.
If you suspect something, get them checked. If you get vibration while applying the brakes, get them checked. If you get grinding noise from the brakes, get them checked.