Yes, it's not unusual for bunnies to get arthritis.....you may have to consider changing the litter box to something with lower sides. I think the House Rabbit Society at www.rabbit.org can give you ideas for coping with an arthritic bunny in their health link.

I too have an 8 yr old chocolate rex and she shows some signs periodically of stiffness. You might give her a bunny massage ...well all over, but esp on her lower back and her back legs. Just easy, circular rubs (image what you would like in the way of a massage and then do that to your bunny). Use a medium-firm pressure but not too hard either.

Good luck, get back to us if you have any other questions.