I offtanked naxx 10man in gear similar to that, so the answer is a definate yes.
You should be able to maintank most things as well, and please remember that for a lot of raid bosses, offtanking is harder (skillwise) then maintanking.
Take Kel'Thuzad for example. The main tank has to keep him in place and avoid his void zones. The off tank has to pick up the adds - coming from two different sides - really quickly, or clothies may get killed. That single moment of picking them up is harder then the entire fight for the main tank, since there is so little room for error.
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