Pulling a pitman arm is one of those things that takes massive amounts of patience and waiting. Go ahead and spray it down with PB Blaster (that stuff is amazing, you can get it at your auto parts store). When you get a new pitman arm remover (you should be able to borrow one from Autozone) go ahead and put it on there as tight as you can (I used a torque wrench for the extra leverage) and spray it down with PB again. Then, when I did it, I hung a drop light off the arm to heat it up a bit. Leave it all like that and every so often go out and turn the wratchet as much as you can and leave it a while longer. Schmo suggested, whack it with a hammer a bit as well. After about a day of leaving the puller on there keeping constant pressure on the arm you should be able to go to work on it again, whacking it with the hammer and pushing and pulling on the wratched as hard as you can until it pops off. By the way, be careful you aren't in the way when it comes off, it can pop pretty fast.