How do I go about getting back into the guard after a surgery from a military injury?
Are you trying to go active or guard/reserve, that will be one determination. Currently with the downsizing and RIF taking place, a prior service, and a medical separated one is not a prime candidate for the military need at the moment. Have you been cleared by MEPS or just your own doctor, is the doctor a MTF doctor or civilian tied to Tricare, there are a lot of unanswered and missing information here for you to get true and accurate answers from potential SMEs.
Did you receive a VA rating after getting out in 2005, and so on and so on, again with the lack of prominent information from you on this matter it is very difficult to provide advice/guidance/insight as to what you could do or who could assist you in your quest.
The biggest issue for you is that 20% disability rating from the VA, depending on what it is for, you may or may not be able to get a waiver for physical from DOD, but again with the current RIF in place it might not be possible, I haven't been in recruiting for about 7 years so not sure what the changes are that have taken place. I would recommend talking to a recruiter or retention NCO that has no stake on whether or not you affect their numbers, I have friend I will suggest you talk to, search SSG Joaquin Goicoechea, send him a message and mention I sent you too him, if he doesn't know the right answer, I am sure he can probably find the right one or direct you where exactly to go.
Good luck and stay safe.