PS, I don't like the idea of 0-5 feet high on each end of the fence. Instead, build the whole thing at 5 feet and plant a nice bush at each end that will soften the transition. The "ramp" is more of a trip hazard, and would be harder to build.
By the way, yes, you do need concrete. Rent a 1 man post hole digger. It has a set of wheels, a motor above the wheels, and the auger is near the handle end, making this the easiest hole digger I have ever used. A concrete mixer rental is worth the effort too, and have your lumber delivered, along with your concrete. Keep the bags DRY until you need them.