or are Shaggy and the gang just really really high?

I'd say that he can really talk. Because sometimes he conveys information to them that they would have had no way of knowing. They're not just making up stuff for to him "say".

I say they're high. I don't remember the other people on the show being able to talk to scooby except for the gang.

D

I'd say that he can really talk, but that doesn't negate the fact that Shaggy is really really high.