What you will need:
-A jack that can jack on the rear differential
-2 jack stands
-small sledge hammer (2.5 lb. works)
-Very level ground
Things you might need:
-a large (8" minimum i believe) C clamp
-lots of time
You will start by chocking the front tire(s)
Then jack up the rear of your truck. (jack under the differential works) You will need to get the tires a good way off the ground as the tires and axle will need to be lowered to allow for the longer shackles. (About 3-5 inches of space between them and the ground should work.) Then place your jack stands under your truck to hold it in place and allow the axle to drop.
Next, you will lower the jack under your rear differential and relieve the compression of the springs holding a bind on the 4 bolts that go through your shackles and leaf springs.
Disconnect your rear shock absorbers from the axle by removing the bolt that holds them in place. (1 per shock absorber) This will allow for the axle to drop sufficiently to install the longer shackles. Otherwise, the shock absorbers limit it's travel.
Loosen the nut on the 4 bolts running though your leaf springs and shackles. (2 on each side) You will need to back them with another wrench or ratchet.
Leave the nut on the bolt so that you can pound the bolt out without damaging the threads. Next is the moment of truth, remove your backing wrench and turn the bolt around inside the bushing, spraying your lubricant as you turn. If it eventually moves freely or easier, then you are in the clear. If it is stuck, I would (and have) reassemble everything and take it to a shop to get it done. (cost me $100) If it is stuck, this means that the bolt is siezed inside of the bushing and is a MAJOR pain to remove. One way I have tried with success is like so:
Pounding on the clamp as you tighten it. This took FOREVER on mine, but worked great. Also, be prepared to destroy the clamp. I bent 2 when I did mine.
After I got mine done by the shop, I examined the shackle and found that it was bent from the inside. I believe they used an air hammer on the shackle itself to pull the bolt through. This effectively destroyed the factory shackles so I would not attempt this on my own.
Once you have gotten the bolts moving, remove the nut and beat the bolt out of the shackle/leaf spring bushing being careful to not damage the threads. Once the bolt is flush with the bushing, you can use the punch to ram it the rest of the way out. You may need to raise or lower your axle via the jack in order to relieve the bind on the bolt and possibly your punch. I recommend doing one side at a time.
Once you have all 4 bolts out, remove the factory shackles. Here is what you should be looking at:
Next, lower your axle to allow room for the longer shackles.
Be careful not to put pressure on your brake line running to your rear axle when lowering it or you may tear the hose.
Line up the new shackles using the jack to raise / lower the rear axle. Run the bolts through the shackles and leaf spring bushings making sure that they face the same way they did before. Tighten the 4 bolts.
Lift the rear axle again until you can reinstall the 2 bolts that hold on the shock absorbers. Install and tighten.
Raise the rear of the truck with the jack under your differential and remove the jack stands. Then lower your truck to the ground.
Your truck should be higher in the rear. To correct this, you will need to lift the front via tightening your torsion bar adjusting bolts.
In order to do this you will want to be on very level ground. Lift the front of your truck (I did mine one side at a time) and tighten the torsion bar adjuster by one revolution:
Repeat on the other side.
Lower your truck and drive it in a "figure 8" then return to the same spot. Measure from the top lip of your rim to the center of your fender lip like so:
The front measurement should be 1/2" lower than the rear to be level. If it is not, repeat the torsion bar turning until you reach the target ride height. The number of turns varies for each vehicle, there is no perfect number of turns.
You are done, get an alignment and bigger tires =D