A couple of years ago we had an emergency and I had a shuttle company bring one vehicle to Echo park, they got it down safely with a trailer. So it's doable, but I don't know first hand what the condition is.
The Rainbow park road is 2wd accessible, just dusty.
But my question to you would be why take out at those locations and miss Splitmountain? That's the funnest part about running Yampa! It only takes about an hour or two to get from Rainbow park to Split Mountain take out.
Also if you take out at Echo park, you'd miss one of the most amazing landscape features I have ever seen. The spine! Also the pliers rapid in whirlpool canyon and the Split mountain itself! Also Jones hole hike....the list goes on