I wouldn’t be so sure that Rush is agreeing with them. Rush could be talking about Castle’s discussion with Bush about losing support for the war, and they could be mistakenly think he’s talking about the impeachment vote.
Rush usually realises when he’s wrong, corrects it, and moves on. I’d be surprised if he was still saying Castle voted for impeachment, when it is so obviously false.
I didn't hear any of the program. But, while Limbaugh clearly understands the big political picture, I have found that he sometimes gets a bit confused in the process that is the sausage making. I could see how he might, because of a lack of subject matter understanding, read a blog posting claiming just this accusation then look at the vote and think, "Yep, Castle wanted to impeach Bush".
Limbaugh, like so many here, might believe it because he wants to believe it.