If your status is a tourist in DR, the 1949 Traffic Convention will apply. How long it will apply is a question of DR laws. As only a nonimmigrant tourist, the signers of the Traffic Convention will allow tourists to "import" their car for the duration of their visa. This is how Americans take a car to Europe and use their State License Plates there for a while.
In the USA, you may import a foreign-made vehicle without regards to smog emissions, if it was made prior to 1973(?). You'll need a US customs broker whom specializes in the importing of these automobiles. (Import a Chinese "vintage motorcycle" of the 1940 design year, but that was still exactly so made in 2001, for example, and you're in some big trouble. )
Such custom broker services is an issue for even US-made cars "exported" by tourists to Mexico and Central America because of unleaded fuel issues. If you go beyond to the known countries without unleaded fuel, you'll be required to make your US-made car conform to EPA regulations again. So coming back is also a reimport issue of a 1973 Jeep, because I think you're right in the very year the EPA standards became effective.
So how about just buying a brand new or pre-owned (used) SsangYang CJ-body "Jeep" from South Korea? Ocean shipping is relatively nominal as is any tax-free export purchase in Korea, and it can be shipped directly to the DR from Korea.
http://www.film.queensu.ca/CJ3B/World/Korea.html
http://www.koreamotors.com/profile.html
Daewoo just went bankrupt and parts are not easy to get. But the resourceful type can bypass this, if they can repair it with used parts, and you'll still save a bundle on these discounted vehicle prices as a firesale item.