Hmm. Kidney disease, only cardio and increasingly skinnier legs, eh? That is a hard nut you've got there. I guess the only solace for you I can think of is do stuff like stair-stepper cardio, really accentuating the step extremes. This is a good cardio exercise and it not very ballistic. I know some girls in my gym and that is all they do, and their legs and *ahem* ass are well toned and have good size. May not be the panacea/solution you're looking for, but it may arrest further wasting, may even build them up a bit. Best of luck and heed your dr's orders.
/look for the stepper that has the steps that go up and over like an escalator.