Scott,
I'm sorry, I can't find the info on the van we hired, it's been a couple of years. I'm surprised none of the "locals" on the board have jumped in on this request.
Maybe an online search of tour guides in Belize would allow you to book something in advance.
I still say you can hire a van at the port to take you wherever your group wants to go.
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Paradise Gallery & Frame San Pedro, AC
"Custom Framing & Art Gallery featuring local Artisans"