The Obama Administration forced Sherrod to resign before Fox News even aired the video. So it would still be an issue.
Could you please get your timeline on this thing correct? You're having quite a bit of problems getting the basic facts down.
Sherrod said her "superior" called her and demanded that she resign before Glenn Beck got a hold of the story, though she did say that the superior did so at the urging of the White House (EDIT:
This article says it was USDA Deputy Undersecretary Cheryl Cook that made the call and who Sherrod says told her it was because the White House wanted it). The White House and Vilsack both say the decision was all Vilsack's doing, without Obama's influence (though the White House did say that they supported Vilsack's decision).
So, while it's technically correct to say that "the Obama Administration" forced her to resign (insofar as Secretary Vilsack is a member of said Administration), your earlier statement that "The White House insisted that she be fired before Glenn Beck aired the story on his show" appears to be, at the least, in dispute.
As for the Fox News thing,
this LA Times blog entry says that Bill O'Reilly taped his show (in which he repeated Breitbart's initial allegations that Sherrod was racist)
before Sherrod's resignation, though it aired
after the announcement of the resignation had been made.
This article posted on Fox News' website states "The Agriculture Department announced Monday, shortly after FoxNews.com published its initial report on the video, that Sherrod had resigned" (ie, after Sherrod had actually resigned, but before anyone else knew about it).
So, it's indeed true that Fox News didn't start reporting on and repeat Breitbart's false allegations until
after Vilsack forced Sherrod to resign. But they were not only prepared to repeat Breitbart's falsehoods before they heard about the resignation, but even
after reporting the news of the resignation they were uncritically pushing Breitbart's version of the speech (as the linked Fox News article above shows).