2002 Mustang Gt will crank but will not drive or reverse
 

If you're sure it's coming from the rear end and not the middle, then there's a problem with the axle. And that's not a good thing(tm). That's possibly the pinion/ring gears stripped, or a snapped axleshaft (not driveshaft, the shafts in the axles from the middle to the wheels.)

If you're just hearing things and it's coming from the middle, then the trans is the issue, and it could be as simple as low fluid. It then escalates to bad fluid, clogged fluid filter, then broken things(tm) in the transmission case. Like valve bodies, clutches, bands, stuck check balls, and/or torque converter.. and probably other things not listed 'cause I'm not a transmission guy to that extent.

It's unlikely to be the driveshaft as if either u-joint destroyed itself, or the shaft snapped, anything to the point that the car doesn't move... you'd kind of notice that right away. The cars are not normally equipped with means to keep the driveshaft from falling out in those cases. So not it.

You might put it in neutral with the car off, then roll it around and listen to see if there's noises happening. You might also put the car up on jack stands under the axle and the front frame rails and put the thing in drive at idle and get under it to listen. I'd advise a friend to help out with that, and being careful shouldn't have to be mentioned here, right? If you can more pinpoint the noise/situation, it'll help estimate what's the problem.

The very best answer in such things, if you're not versed in either the axle or the transmission, is to take it somewhere to diagnose and fix. And be prepared for a semi-hefty price to fix 'em.

Hope that helps in some way. Good luck, and welcome to the forums! :nice: :wave: