I can't remember, how old is Rocky? Could there possibly be some arthritis in his hips? I've read that something like 80-90% of cats over the age of 12 have some degree of arthritis, usually in either their elbows, shoulders or hips. Maybe Rocky would benefit from a glucosamine/chondroitin supplement such as Cosequin. It's very well tolerated by cats and comes in capsules that are easily mixed with food. Aztec has been taking it for a few years now and it's definitely made a difference (less limping) despite some pretty severe arthritis in his elbows.
"To close your eyes will not ease another's pain." ~ Chinese Proverb
“We must not refuse to see with our eyes what they must endure with their bodies.” ~ Gretchen Wyler