I can't tell you if it's diabetes-related -- it's never happened to Motska. But if it gets to the point that he's not grooming himself at all and not eating, please take him to the vet right away. When I was a kid, we came back from vacation to find that our Tiger had not been grooming himself and he was a little skinnier and we couldn't figure out why. (We'd been having a neighbor come over to feed him and let him in & out of the house a few times every day.)
A couple of days later, we realized that his tip of his tongue was cut and barely attached at one side! (Sorry to gross everyone out!) He had apparently gotten in a fight with a local feral cat and gotten his tongue cut. One trip to the vet and two stitches to "tack it" together, along with some liquid food fed by syringe and he turned out just fine.
I doubt that this is the problem with Peaches, since it's just his hind legs that he's not grooming, but if it gets worse, make sure to take a look (or have the vet take a good look) at his mouth to make sure that everything is ok there.
In the meantime, (((hugs))) and prayers for you and Peaches!
Kris