Ok, so it's a 1998 mazda 626. I was driving and heard what sounded like something falling off the car and scrapping while driving on the highway. I looked under the car nothing is out of place. I drove it home and thought maybe a brake pad broke, did the brakes and it didn't fix it. I can put the car on jack stands with tires off and if I put it in gear it sounds like a terrible loud grinding coming from the wheels/behind motor somewhere in that area. I won't even drive it as it is. Any ideas on what i could be? I'm not familiar with mazda cars.

 

My guess would be a wheel bearing. If so, it'd grind, maybe rhythmic, in forward or reverse.

 

constant loud grind whenever it is in gear and the wheels are moving. It is nasty and loud.

 

Anyone check the half-shafts for a bad CV joint?

 

That's a possibility, they can make some bad noises too. check the cv axle boots for tears, both inner and outer. Most of the time a cv acts up, a boot has torn and dirt and other contaminants get inside, screw up the gears eventually. Easy way to tell if the outer is going bad is clicking when you turn.
I wouldn't rule out wheel bearings though, they're always nasty and loud when they're going out. Can be very dangerous too