Jack the rear end up (jack under pumpkin) so that you can put jackstands under the rear framerails, you'll want to go as high as possible so that when you let the jack down (tires to the ground), your suspension will go to full droop. This should unload your spring.
Take off the tires to give you some working room (jackstands still under framerails) Remove the spring eye bolts (I recommend a large sledge hammer, but try to not damage the end of the bolt, use a block of wood or something to deaden the blow). You'll probably have trouble with your lower shackle bolt, again BIG HAMMER.
Make sure to re-assemble the shackle to the spring with the bolt going from the inside of the frame pointing outboard (before assembling spring to vehicle), this will keep the bolt from hanging up on the shackle bracket during articulation.
Hope this helps!