maybe the axle splines?????

I seriously doubt that it would be the axle splines. Damaged axle splines wouldn't keep the vehicle from rolling backward down a hill, they would only cause problems with the differential. Chances are the emergency brake cable is old and rusty and thats why it took some time for it to start rolling. As for the popping noise, describe it a little better. When does the noise happen? Does it always happen under those conditions? Is it intermittent? It might be easier to help if we knew if the car was and automatic or a 5-speed. Have you done anything with the rear end? Have you worked on the rear of the car any?

Dude I would suggest that you remove the rearend cover as soon as possible. It sounds like you are loosing your carrier pin which will destroy your rear end when it comes all the way out. It is the pin that holds the spider gears in the carrier, it is 3/4" in diameter. usually what happens is the little bolt that holds it in shears and allows it to start to migrate out. . good luck

How about when turning a corner?

I would jack the rear of the vehicle and support it on jack-stands. Then remove both rear wheels and remove the brake drums. Look at the return springs and also look to the parking brake cable to see if they are applied. It is possible the brakes are bound up on one side (broken spring or star adjuster that is wedged) In addition try rotating the drive-shaft by hand and see if you can duplicate the noise. It is also possible the noise is coming from the carrier. You can should remove the differential cover and look for signs of broken or loose parts. Again rotate the carrier and look for the noise. To find the root cause it takes a diagnostic approach and the process of elimination. I would start with the brakes first and then look to the carrier. You do not want to remove the cover and lose all your fluid and have to reseal the cover if the noise was in the brakes all along.

it's probably the brakes sticking, if the drums are hard to remove that should be your first sign they're stickin, good luck and odnt forget to take off the E brake when you pull the drums off

~Mark~