When you leave the 5 boroughs and drive East, there are a few choices you have. You can take the Southern State Parkway, the Northern State Parkway or the LIE.
Depending on where you want to go, one would be a better choice than the other 2. The Northern State and the Southern State end earlier on Long Island - meaning you can't drive as far on them, because they end in urban areas. Also, if memory serves me correctly, there is no rural areas being passed on either of them. It's all high density, lots of traffic and lots of population along those highways.
The LIE is much the same for most of it's length. HOWEVER, there is a big big difference in environment, population, traffic and you can actually see trees when you get to the end of the LIE. The high intensity urban sights start dropping off after Stony Brook, which only leaves a few exits before the end of the LIE. So, if you're just driving looking for some forest/woodsy areas along the LIE, you won't really find anything useful until around the Stony Brook area (around exit 64-65) and even then you would have to know where to look. It's still quite populated there.
If you continue on the LIE towards Riverhead, you will need to pass out of 64-65 and then into 66 before you start to see the scenery changing enough to actually start looking realistically for a place to dump a body. It would make sense that you would go an additional couple of exits to really get into the best "rural" area you will find along that stretch. So, starting around the Brookhaven Lab (exit 67) if you know that area and Manorville (exit 70) where it's probably the most un-populated yet, starts to become more of a desirable dumping area. If you're really lazy, you will just drive to the end of the highway (exit 73). There's nothing there.
It's so much of a difference that there comes a point after Stony Brook that you can't even believe you're still on Long Island - that's how much of a difference there is in environment. And if you go too far off the highway East, you're into the Hamptons and Montauk and back to mass populus again - not the most condusive to body dumping long-term. So, in a sense, you only have that bit of area between 70-73, unless you want to drive non highway roads NorthEast to Orient Point - which would take a long long time.