We just found out about a van shuttle service for ground transportation options in Belize. One of their pages seems to date from Nov 2010 but they are fresh news to us.
Belize Shuttles and Transfers has rides starting at $35 (plus tax) for a regularly scheduled trip from Belize City (or airport) to San Ignacio as well as many other common destinations in Belize, Guatemala and the Yucatan. They also offer private van transfers (think taxi for up to 3 people) to all those destinations as well ( and probably others) with prices ranging up to $325 (plus tax) for a ride to Cancun.
Belize Shuttles and Transfers claims that all their vehicles are air conditioned vans or SUVs. We have no personal experience with them, and checking their Trip Advisor reviews, the only negatives were back in 2010 and they may have gotten the wrinkles out.
Here is their rate sheet.