Or you could just give it IVP and then reduce the dislocation. Much more easy and beneficial in that situation.
Otherwise, initially your goal is to reduce the pt's pain...that will be accomplished much faster if the drug is given intravenously vs IM. Your concern is valid, but, (and this will depend on your local hospitals) if you're giving morphine as your analgesic, then if you give a dose as you arrive at the hospital, they'll most likely be ok. But, if where you go has a history of extended waiting times for in-hospital care, then it might be more appropriate to give a final dose IM, as long as the pt needed it and could tolerate it.