It is unfortunate, but a raiding guild will probably have at least the minimum tanks needed to get some content done. And they do tend to be long-term members, or dedicated or officers/GM. There are exceptions, I know...
At your gear level you'd be best served either getting into a low tier raiding guild, one that has more porous positions due to the casual atmosphere. You could also try applying to a regular raiding guild and sit on the bench until a spot opens up and you can slip in to show em your awesomeness.
Either way, learning and gearing up your dps side will only allow for a greater opportunity.
"I'm a rage-a-holic! I'm addicted to rageahol!" -Homer Simpson