"Remember that guy off in whose trailer they were jacking?"
Y'know....that's actually completely incorrect. "Off" isn't a preposition, so the person who wrote that line for the movie really fucked it up. "Of" is a preposition - remember when people used to say "off of"? Yeah. "Off" just kinda wiggled its way into our regular grammar, but it isn't really right.
Regardless, since "jacking off" is a phrase in itself, it shouldn't be split.
Technically, it should be "Remember that guy in whose trailer they were jacking off?"
But whatever. I'm a spelling and grammar nazi and I respect people who are able to use the English language intelligently. Talking is one thing, but if you're going to write, you might as well do it right. Note that this doesn't extend to writing natural dialogue.
Sorry.