I've got a 5 month lab and my vet has recommended not to let her jump in and out of cars and not to take her on long walks until she is at least a year old because labs can have dodgy joints....A cod-liver oil tablet a day is good for joints and a shiny coat!
The stuff used for bacterial claw problems in cows is really good - there is a green spray and a green gel which you can buy online. It is antibiotic free and contains cooper (which kills the bacteria) and zinc (which helps with the healing) and so it gets absorbed by the skin and gets to work...
Have you tried Hoof-fit gel? They sell it for digital dermatitis in cattle (basically cow mud fever) but is really good on mud fever - it actually helps to treat the cause rather than just being a barrier. I think it is about £25 and you can get it online. Only down side is that it is green!