From Faq
The "diff drop" modification lowers the front of the differential down 3/4" to 1", in order to improve CV joint angles. The angles of the CV joint increases as you lift the front of the truck via top-load/spring-load spacers or aftermarket lift coilovers. The increased angle will cause the fins of the CV boots to come into MORE contact than they were designed for, eventually causing the boot to tear or rip. A torn boot will spew grease throughout the wheel well, and the absence of grease in the boot will ultimately cause the CV joint to fail. Inserting a spacer between the frame, and forward differential mount in theory should reduce the amount the fins are touching. There are many sources for differential drops; Cornfed and Revtek being the most widely used. The installation procedure is the same for all manufactures.
Tools Required: Socket set (19mm, 12mm), 19mm open end wrench, large drill bit or tin snips, blue thread lock.
Parts supplied: 3/4" - 1" spacer, 2 longer grade 8 mounting hardware, 2 longer 12mm skid plate bolts.
Remove your skid plate: 7 12mm total attach the plate- Three across the front, two in the middle, and the two rear most. If you remove the other two bolts the plate will come apart in two pieces, which is fine.
Remove the stock diff bolts: 19mm open end wrench on the top nut, and 19mm socket on the bottom bolt. Remove both bolts and set them aside. The front of the diff will drop down.
Insert spacers & new bolts: Insert the diff drop spacer between the top of the diff mount, and frame. Insert the new longer bolt through the diff mount, spacer, and frame. Put some thread lock on the nut, and start it on the bolt (now sticking trough the top of the frame). Once you have both bolts loosely installed, tighten them down with your 19mm open end, and socket.
Trim skid plate: To make the skid plate fit once again, you will need to trim or enlarge the rear most mounting holes by ~.5"
Reinstall skid plate: With the rear most holes enlarged, reinstall the skid plate using the two longer 12mm bolts in the middle mounting points. Loosely thread each bolt until you have them all started. Tighten each one down.
You're done!
Many people claim success with this mod, while others find that it does not help enough, or at all. In fact, most people will tell you that it is useless now. Because the spacers only lower down the front of the diff, and the output shafts to the CVs are somewhere in the middle, the spacer does not directly drop them- thus having much less effect as the spacer height. There have even been an unlucky handful who have had the rear differential mount shear off because of the diff drop. Some other side effects include: change of pinion angle, and decreased clearance under the skid plate.
In the end, diff drop, or not, the boot is still going to tear. It is just one of those things to keep an eye for on our trucks. Besides the diff drop, the only other preventive measure is to keep the boot free of dirt or sand. Dirt or sand between the fins will act like sand paper, eating the boots away with every rotation. Rinsing the boots off as often as necessary will make them last a bit longer.
original write up
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