I think mg is correct. The only proof any of us have is what Rush has shared about the topic on his show. I haven't heard him talk about his draft status for some time, but he's been rather open about it.
First of all, he was on the "young" end of the Vietnam draft as he was born in 1950 or 1951, making him a 1968 or 1969 high school graduate. Secondly, he had a college deferrment for awhile because he did attend a couple of semesters of college (I think he said he did a fall semester one year, didn't complete it but tried again the next fall). This puts him up to 1970-71 when the draft was ebbing ("Vietnamization" was the buzzword from Nixon and Kissinger).
Lastly, I think you are right, and he did have a medical reason but I thought it was related to something like a football/knee injury. Anal cyst is as good a reason as any....except in a more militant time, they would have fixed him and sent him anyway.