Just finished installing a 4" Skyjacker lift on my '76 Ford F250 4x4. Everything went well but after a few runs up the street and back a very loud and repetitious 'ching' ching' ching' sound developed. Only happens going forward, not reverse, and even with load off the driveshaft. The noise includes the hollow sound of a driveshaft, but could be occuring in the diff and resonating up. Not sure. I then noticed the pinion angle is pretty close to dead straight on with the driveshaft, but the angle to the transfer case looks acceptable. The two angles definitely do not match. The angle of the whole diff seems pretty extreme to me, making me wonder about pinion bearing lube. Seriously doubt I drove it enough feet to make it dry up and go south already. The rear springs came with shims already bolted on. After only a few hundred feet of playing with the problem, the noise is getting louder and extremely consistent. No sounds prior to the lift.

My thoughts are:
Pull it apart and take the 1/2" shim out entirely, or use a 1/4" shim.
Replace the rear u-joint - maybe the zero degree angle is just too much for an old u-joint? The u-joint feels tight, but I'll block up the rear end and play with it some more tomorrow.

What is your best guess on this one? I searched the forums and get all kinds of varying opinions on diff angle - does it matter for pinion bearing lube? Can the diff be angled up a bunch? We're not talking about an irritating vibration here, its more like a bang bang bang. I don't put enough miles on this truck to worry about u-joint wear. Should I just try a new u-joint or is the lack of pinion to driveshaft angle going to make a new u-joint pop too?

This cost me a hunting trip tomorrow 'cause I did't want to get stuck in the woods... until a few minutes ago when I realized I could have just dropped the rear driveshaft and ran off the front. Now it's too late and I'm too tired...

 

4 inches is hardly any lift at all, and should not have any ill effects on pinion angle, so my thoughts are that the 1/4" wedge is too much. That pinion should not point up that high with such a limited lift. While I do not believe that the pinion is dry, I do believe that there is a worn / dry joint that is causing some trouble. If the angles of the two joints are considerable dibberent, you will have different joints speeds and can cause some trouble.
You may also look into the posobility that the slip joint is dry, and the slight movement of the splines in the joint is binding causing your popping noise.