Jacking up the frame will unload the springs. I'd suggest supporting the frame just ahead of the front of the springs on a couple of sturdy jack stands until the tires just barely clear the ground, and using a rolling floor jack under the center of the rear end to manipulate the rear end up and down to keep it from doing anything dramatic as you unbolt shackles, etc. Leaving the wheels and tires on will prevent damage to the flex line for the brakes, parking brake cables, etc. Even if you have to unbolt the U-bolts and take the springs to a press, you can do that one spring at a time without doing a complete rear end removal.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!