I just changed mine about 2 weeks ago, and it wasn't fun My dealer said to use a pickle fork and an air hammer .........yeah right!
First loosen the box and large nut on the pitman arm, (don't remove the steering shaft) and point the pitman arm shaft towards the engine. Then buy a pitman arm puller from a parts store (its easy to identify the one that works the best. Their relatively short and have 2 ears with a large center bolt) Now for the fun part..... using a grinder or die grinder cut into the pitman arm towards the center splined shaft, but of course don't cut too far. Then when you have cut as much as you dare put the puller on it and it should come off. If yours has been changed recently then skip the grinder part, but if it was like mine you will just brake the puller (I broke 2 ) if you don't cut into the pitman arm. Good Luck! also look for the thread I posted about 2 weeks ago I't might help you some more
P.S. if you plan on keeping your truck then Anti-Sieze will be your best friend for re-assembly so it's not such a pain next time.