I highly reccommend JJ and Rocio at Welcome To Paradise Transportation. Last year JJ and Rocio drove us from Santiago to Samana, then a week later from Sosua to Santiago. They are fair, reliable, and really nice people. Oh, and safe drivers too!
You can even make reservations online.
We plan to call again when we go to the DR this year, and this time we're bringing Mom. Recommended to us by Hillbilly himself.
Thanks HB!
Julie