I used a long bar with a pipe on the end (maybe a total of 3 ft) and a whole lot of force to remove mine. Sounds to me like you're on the right track with what you're doing.
I can't find it right now, but I think there's a thread on here (here it is: http://www.subaruforester.org/vbulletin/f89/stuck-rear-diff-filler-drain-plug-38690/) where someone mentions using a jack to slowly apply increasing force on the end of the bar. Also someone has mentioned to try tightening them first and it might break them loose. Sometimes the sharp impact of a hammer on the bar will break things loose, but I realize there's not a whole lot of room under there to swing one effectively.
Just a couple things to consider:
1. Are you turning them counterclockwise? (I know, it sounds stupid, but I've done it before)
2. Make sure you undo the top plug first no matter what...don't want to have the diff drained and no way to refill.
I have found a mixture of the previous post's ideas work. Use a jack or a friend to put pressure on the plug. I use a breaker bar and a pole. With steady pressure on the plug, hit the breaker bar on the head like you are trying to hammer the plug into the hole. Sometimes just tightening it first works too.
Gee-forgot to post back! So, I got the plug out by rolling the rear wheels up on a few pieces of 2x6 just to get a few inches higher. Sprayed the plug with PB Blaster, put a breaker bar on it, and used a jack to move the bar. It still took a lot of force. But it worked well. Thanks for all the suggestions!