Grouping in Everfrost could take you to 50 classic, obviously higher with Kunark.
It would probably be rough when you make the jump between killing mammoths and starting on ice giants though. Not sure if the icy orcs drop raw-hide or ringmail as their best armour, but it will be scary facing giants in that armour at ~lvl 36 with only shaman to heal you
[You must be logged in to view images. Log in or Register.]. Unless there is a camp in the ice caves you can do after levelling on orcs/mammoths but before giants?