I don't know what differential surround is... give me a link.
True rear surround channels, sometime expressed as matrix, L-R, or Dolby rears, can bring the front image up to where the dash meets the window, even with low front door mounted speakers. Everything needs to be set up just right.
Delayed or non-delayed mono or stereo signals in the rear probably won't do it. You might have some success with delayed, but that's not the best approach in my opinion.
Oh, maybe by differential, you mean difference signal.... that's also a name for rear surround sound channels.
I did design the L-R ambiance channel for the old JL audio 5-channel amp (I think it's discontinued now). I know a couple of things