There's a little plastic guard (I guess that's what it's called) that goes between the fender and bumper. Do you absolutely need to have it between there?

theres a little plastic guard? I dont remember a piece between mine, and there is definitely not one between my fenders and bodykit, so I would say no its not ABsolutely! necessary, but if at all possible to keep it on, i would keep it, as it should preform some purpose.
EDIT-
Are you talking about the long plastic piece that runs on the front bumper (although i think this piece is metal from memory) that has the three threaded holes to bolt on the bumper?

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Yeah that's it. I dunno about yours but mine is plastic. I ordered on form Toyota but I finally got all the parts to fix the front end yesterday, so it'll be done tomorrow. I was wondering if it'd be fine to roll without it until I get it from Toyota.

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I don't think you absolutely need to have it, but I'm sure it would deture rust, as well keep water away from crucial components in the bay...