I forgot to say that this doesn't happen in 3rd gear. It only happens if I'm in overdrive(4th). Any help would be greatly appreciated.

This sounds VERY simialr to a problem I had with my '90 Legacy. I took it to a shop and they thought that it might be a burnt valve and would cost 800 to fix!! But then I took it somewhere else for a second opinion and he said it was the fuel pump of all things, and I got a new one and voila! Fixed. Only thing is with mine it didnt die if i didnt back off, but instead the car would shake..badly. But IDK, it might be worth a look to have your shop look at your fuel pump. Hope that helps.