I'd do what Thinus says. Pick a key, I use K, bind it to forward on your tank, backward on your hunter. Then, click on a badguy. Keep your hunter on follow of course, and as soon as you get within range, use K to approach. Your tank will walk up to the mob and aggro, the hunter will back up and be in range.

Just make sure you use K as the alternative forward/backward, or else it'll unmap your W key.