Replacing bad U-Joint

I am trying to remove the drive shaft of a 91 f150 so that I can replace a bad U-joint. I am having a problem removing the four 12mm 12 point bolts attatching it to the rear.

This is what I am trying to get off: http://neptune.spacebears.com/images/stories/scdrb400.jpg

Whenever I try to break the nuts, even with a long lever, the drive shaft moves. Does anyone have any remedies for my problem?

thanks

Have you tried placing the tranny in the park?

Or locking the transfer case!