I would suggest you make sure the GM/Raid Leader explains to the guild/raid that you are being asked to do this to help the guild. The guild should understand that tanking has a steep learning curve to get good. I ran boomkin/resto in vanilla and had the same issue when BC hit. I actually started tanking in Kara with quest tanking gear and it was painful for me and the guild. I worked hard to upgrade skill and gear a lot and once I had more exp it was easier for me and the guild. I would also suggest running as many 10 man older content that you can as a tank. Go to Naxx and run a wing or 2 and saph/kel. Do maly/sarth/toc/togc/uld HMs/voa these will all give you exp tanking different bosses with different mechanics. That is the real key. You have to get comfortable with how to tank not any specific boss strats.
Feral Druid bear/cat