Re: Got cats?
Can't help with the diabetes question, crystal -- save that I know from watching the vets that they inject into the scruff and the cats don't seem to feel much at all (but they could be so traumatised by the trip to the vets in the first place...)
Stormfeather, are the boys entire toms? If so, that's your problem. We've always had neutered toms and never experienced any scent-marking. If yours are neutered then it could be a stress issue -- we use that feliway stuff on eg Bonfire night, which seems to help a little.