Hi again, Tabatha, and thanks for your reply, your patience (I had a lot to say) and for welcoming my thoughts on your cat's problem.
The first thing to consider, even though your cat's diabetes is being controlled with diet now, is diabetic neuropathy. This is a neurological condition in which nerve impulses are weakened, due to the diabetes, and is common in older, diabetic cats. I think you'll find these webpages on feline diabetes, informative:
I found the FDMB (feline diabetes message board) very helpful, when my cat was first diagnosed, and in the years, since. You can talk to many other cat parents who are going through the same things as you, and find out great information and get support. www.felinediabetes.com
Another possibility is a condition called Vestibular Disease, also, neurological in nature, and which causes the appearance of a 'drunken' walk, or stumbling, unable to balance well. This condition adversely affects equilibrium and and is more common in older cats, as well. Many times, Vestibular Disease will resolve on it's own, after several weeks, but if not, it does need to be treated by a vet, to minimize the symptoms of dizziness and possible nausea caused by the dizziness.
I'm assuming your cat does not have access to antifreeze (outside or in the garage), but ingestion of this very toxic substance can also cause a drunken or stumbling walk, which resembles Vestibular Disease.
An inner or middle ear infection can also adversely affect the equilibrium, so her ears should be checked carefully for any signs of infection. Typically, ear infections are not treated with broad spectrum oral antibiotics, but antibiotic drops, directly into the ear.
Because your cat is older, she may be getting arthritic in her hind legs, as well as experiencing weakness from the diabetes, so take that into account. This would especially be possible, if she is overweight.
I'm glad you're considering changing vets, since you don't feel you got the answers you needed from your present vet, so try to seek out a feline-only, well-recommended vet in your area, if possible. A referral to a veterinary neurologist would be a good idea at this time, since her hind leg weakness may be related to a neurological condition. Specialists are usually found at 24 hr. ER vet centers and Veterinary Colleges.
Re: the urinary infection--it shouldn't cause hind leg weakness, but if she has discomfort in her bladder or urethra, it could cause her to walk in a more 'tentative' manner.
I hope I've enlightened you to a few more possibilities, and wish you and your furry girl much good luck when you go to a new vet and/or specialist! Please let me know how she's doing, if you can, and I hope all will be well!