The vowel sound in words like "behind" (rhyming more-or-less with current pronounciation of "wind") is the older form, if I remember correctly; it persists in some Northern English and Scottish dialects. It's really a bit more complicated than choosing between a long and short vowel; we were taught a "re-constructed" Middle English pronounciation in my student days, and my limited memory of it would perhaps represent the rhyme as beheind and weind, though I'm afraid I have forgotten the phonetics. Poetry has tended to retain such rhymes well after their decline in the spoken language, which does lead to confusion sometimes.