Is it possible to join the National Guard after being med retired?

No if you're medically separated and receive disability you could serve in the NG. But if you are retired you cannot serve in the Reserve Component (except for when retirees are temporarily pulled out of retirement)

I don't know your RE code or the details of your retirement or injury but I seriously doubt that will shake out for you. Unless you were placed on the TDRL and your issue has been resolved I don't see how it would be feasible administratively or physically. But if you want to entertain it you can always contact a recruiter but once they look at your DD214, RE code and get a full rundown of your service history and what caused your med retirement I am 99% sure they will give you the red light.

I'm sure the regulation master will be along shortly, but quick answer, no. Your RE code is non-waiverable.