My car drives fine and everythings great. But when I put the car in reverse and start rolling backwards REALLY slow, my car (rear-end i think) will make this thud.....thud......thud, everytime the wheel makes a full turn. What's the deal?

 

probably the crush collar on the pinion gear. Or the carrier might be loose. Example, going forward might pull it together and going backwards move it apart.

 

I think it is in your reverse gear in the tranny. If it goes forward fine, it should go backwards fine as far as your rear end is concerned.

 

Do you have an auto. or a manual. If it's an auto it's probably not in your trans., the reaction gear set is used in forward and reverse. If it is only tire speed related then check brakes, caliper to wheel clearance, or anything that rotates at tire speed. In your rear gearset your ring gear goes at tire speed and pinion goes 2.73 - 3.23 the speed. If it happes 2 or 3 times during one tire revolution then possible pinion problem, but unlikely. Did you recently have a brake job, change wheels, use your parking brake, maybe it's sticking. If you have drum brakes and the drums have been machined, have them remachined at a very slow cut. Hope this helps.