My family and I will be in Placencia for a month beginning Feb 08. We're hoping to go to Tikal, is is possible and/or feasable to drive there? How long would it take?

I don't know how long the drive is but I wouldn't go there by car.

because of safety reasons?

Yes, I don't think it is very safe to travel by yourself in the country but I may be wrong. Check that with a travel agency. They probably have more info.

To drive is perfectly feasible - it would probably take about 8 hours though.

Better to fly via Belize city - you'll be able to get good value connecting flights - try Maya Air or Tropic Air which both fly from Belize City to Flores.