Depends on the context but in most they are interchangeable. Personally I would use shake off when there is something on my person and get rid off when it isn't. I need to shake that dust off my clothes I need to get rid off my debts. I need to get rid off that bad smell.

Well, it is "shake off" and "get rid of" (not "off"). "rid" here is used as a noun, meaning you are effecting a "removal of" something, quite different than the adverb "off" in "shake off". Doing a quick search, I see some usages of "get rid off", together with some questions and controversy about it. Personally I've seen "get rid of" bazillions of times, but "get rid off" for the first time in this thread. It is also nonsensical structurally. I bet it is a wrong usage arising from confusing "of" with the "off" in "shake off" and others. It could also be an intentional slang to make it funny. Either way, it is certainly not standard English and using it will just make you look ignorant and uneducated. I advise you to get a firm grasp of the correct expression "get rid of" and forget about "get rid off", if you care about using correct English.

Alguna veces es lo mismo pero, shake it off signifie olvídalo. Shake off the cold=get rid of. Librarse de este infermo. Shake off the loss. Olvida la perdida.