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The apple juice is the moisture the ribs will use to kinda baste themselves while in the foil. I think you could use a different juice but you may get a different flavor with it
I use 2 parts apple juice to 1 part beer. Put in the fridge over night, with onion slices, smashed garlic cloves, whole jalapenos, and black peppercorns to soak. Next day when I smoke then foil, use to braise.
After they're done, drain the juices from the foil, and mix in to my BBQ sauce. DON'T WASTE THE JUICE!!!!! Especially after pulling apart a butt. Mix the juice with vinegar from pickled jalapenos, and use as a finishing sauce for pulled pork.
I've used apple, cherry and orange all with good success, and different effect! It's good to experiment, I usually play with only one variable at a time though, to try to keep track of the results.
The last time I smoked ribs, I used a combination of Dr Pepper, apple juice, rum, and some apple cide vinegar for my braising fluid and it came out great. I had read about the Dr Pepper on here awhile back and decided to try it.