You have jack stands correct? If so, the process is easy. Jack it...throw stands under both sides. (place just forward of leaf spring box) I removed tires/wheels to lessen the weight...to each his own on that one. Put the jack under the diff to give some support to lessen stress at times. Remove both lower shackle bolts, one side at a time. (don't try to completely replace one shackle...then work on the other...not fun.) Drop the jack holding the diff a little to give some room for the length difference of the new shackle. Now work the upper shackle bolt loose. There is a weld nut in the frame rail...be careful. The only way to get penetrating oil to that bolt/weld nut area is to remove the rear bumper. Remove bolt, replace shackle and reinstall bolts loosely. Do not fully tighten until vehicle is back on ground supporting its own weight.
**That weld nut can make for a long day if you break the weld. That is a long story all in itself...good luck!
