I was wondering... what was the true purpose of the long necks of diplodocids? Because their necks weren't especially tall like the brachiosaurs, it couldn't have been used for browsing. I know you probaly don't have a definite answer, but what's your speculation or ideas?
Also, how could a massive animal like a Sauropod rear on its hind legs without toppling over?

As far as we know, diplodocids -- like all other amniotes -- habitually held their necks elevated. See http://svpow.com/2009/05/27/sauropods-h … e-rabbits/

So: browsing, the same as brachiosaurs, and all other sauropods. No doubt some also used those necks for other purpose -- for example, apatosaurs may have used theirs in intraspecific combat -- but they were primarily about getting as much food as possible, as quickly as possible.

Different sauropods would have reared with different levels of expertise. Diplodocids were probably very good at it, as their centre of mass was already close to the hips. Brachiosaurs would have done less well, and probably avoided rearing except when absolutely necessary.