I'm not a fan of multiple actions, but the following might work according to a quick check of WoWWiki:
Code:
#showtooltip
/castsequence [mod:shift, stance:1] Rend, Thunder Clap
/castsequence [mod:shift, stance:2] Piercing Howl, Thunder Clap
/castsequence [mod:shift, stance:3] Piercing Howl, Pummel
As for more macros, check out BindPad. And on another note, you could simplify this macro simply by binding it to shift+* and dropping all the mod:shift conditions.