Hi Realxeption,
If you go to the homepage of Negril.com you will find a tab called "transportation" click on that and it will show you a list of recommended, tried and trusted drivers that are all fully insured to carry passengers/tourists. They (I think) have either an email address on that link or in some cases a website. In my opinion, the best thing to do, get a couple of email addys from the transportation link and ping each one an email to see who can cater for your needs. The ones I have come across are very nice and helpful.
Hope this helps you a bit. Hope you have a fantastic time in Negril!!