I've not made the trip, but I understand the road near Sosua up into El Choco (where Casa Linda is) eventually makes it's way to Jamao. The pavement ends at Haciendas El Choco about 5 minutes in from the highway and the road remains reasonably good for at least the next 10 minutes. I suspect it deteriorates pretty soon after that.
Anyhow, that might be the first part of your trip. A while back I traced the road quite a ways using google earth, but didn't try to follow it all the way to Jamao.
So... that 'might' be the first part of your trip. Make sure to get more certain info than I've offered here before you take off.