Simple: Have a druid on your pally account for PvP, keep using the pally for instances?

Thats what I do.

Non heroics are also easy enough like this, but the druid is less well geared, so heroics are tough (and I haven't practiced enough tbh).

See below some related posts:

Boomkin+4xToW = Fun, fun, fun!
Really impresssed with Boomkin DPS
4 shammies and a ?

I started out really happy with adding the boomkin to the team, but was ultimately dissapointed, but others have had more happy experiences.