I did this a while ago and wrote up my approach:
Differential Mounts: Follow-up
I tried the air hammer to knock the old bushings out, but I could see that I would end up boogering up the subframe. The problem is that the bushing shell is very thin and you can't get a good surface to hammer against without banging the subframe too.
The push/pull tool is the way to go. Wait until you hear the thing pop when it breaks free! They are in there very well and you really need to apply even pressure around the entire diameter to get it out without doing other damage.
Hope this helps.