running the speakers directly from the HU would make an audible difference but they wont be performing at their best, say you buy a pair of speakers which are 60wrms each, the Hu would only be giving them 15-25rms each, thus they would be underpowered and not perform to give you what you're looking for, it is advisable to also buy a two channel amp if you want to just change the rear speakers, would make your listening experience much better.
i say buy a 4 channel amplifier, put in some nice budget component speakers in the front a sealed sub in the back and enjoy
otherwise just buy a pair of 6.5inch speakers for the rear, pioneer 6902 have some good reviews about them. connect them to a 2 channel amp and enjoy. and if you realllyyy dont want to put in an amp, just connect them directly, the musical environment in the car would change but wont be at its best.