Mild lameness, occasional fever, lameness progressing and possibly rotating from limb to limb all sound very like one of the tick-borne diseases, sibermom14. Granted we have a lot more ticks here than are typical for Ontario, but I wonder if a course of doxycycline might not be in order? In addition to Lyme's disease, you also have to check for ehrlichiosis, anaplasmosis, and possibly even babesiosis or Rocky Mtn Spotted fever--those last two are emergent here. Also, it's possible to test negative for Lyme's disease and still have it.
We've also seen similar symptoms when our dogs have had leptospirosis.
If your vet is stumped, would he be willing to give you a referral to a veterinary teaching hospital near you? VTHs tend to see a lot of cases of diseases/conditions that regular vets don't normally see at all. We've always had good luck with referrals--the staff at the VTH has always been able to find the correct diagnosis.
I hope you get some answers soon and your dog gets better soon!
Please keep us updated on how your husky is doing