The past tense of sneak is sneaked.
No, it is not snuck, except in dialogue, or in certain kinds of first-person narrative. (The technical term for these cases is “privileged speech,” in case you ever need to win an argument with your copyeditor.)
The philologist in me is fascinated by the fact that an English word appears to be in the process of changing from a weak verb — one where the past tense is signaled by the -ed ending — to a strong verb — one where the past tense is signalled by a change in the root vowel — when usually the process goes the other way around. But the writer in me simply bangs her fist on the table and says that snuck is wrong, dammit, and that’s that.