Do not know if u can still get a ferry from Sabana de la Mar...it would be the quickest. Otherwise you need to drive back thru Sto. Dom. and take the autopista up to Piedra Blanca thru Maimon, Cotui, Pimentel, Nagua and onto the penninsula (or autopista to SF de Macoris and on to Nagua)...and it's about 10 hours.
Other option is to fly from PC to Portillo airstrip, taxi to town and rent a car there.