Driveline vibration out of now where! Help!

I will take a shot at this.

So you just finished putting on the Kelsey Hayes front disc brakes and all was well before that. To start with did you put in the proper "Distribution Valve with the Metering Valve" for the disc brakes? If you still have the drum brake Distribution Valve you need to change this out to the proper one for you discs in front and drums in rear setup, that includes the Metering Valve part of it to regulate the pressure to the front disc brake part of it. The Ram Man on youtube explains this well, Mopar Brakes.

Here is the deal with Discs up front and Drums in the back. Drum brakes take like 300 psi to activate and the front disc setups take like 3,000 psi. So if you don't change the distribution valve the rear brakes are coming on first and hard possibly causing your chattering from your brake drums, and the front disc are doing almost nothing when trying to stop with the pedal real hard.

OK, next down the line check your outer axle wheel bearings on the rear end to see if one them is binding up so much that it is making the Clutches in the Sure-Grip Differential chatter, because one wheel is trying to spin faster than the other. See if one side is getting hot when you take it for a short drive. Possibly one rear drum is set up tight to the shoes and the other loose causing the Diff Clutches to chatter because of the same above effect.

Kind of sounds like one axel in the rear end is doing the driving and the other is not for whatever reason causing the Diff Clutches to chatter. Brocken Parts in the Diff, Broken Axle although you woud probably hear additional contiunuous noises with this, bad outer axle wheel bearings. Sounds like something is not right with the axles and or the center chunk.

It's kind of like a Posi-Traction rear end going around a corner without the special additive in the gear lube, put the additive in and the chatter goes away.

Mopar recommends this same type of additives in their Sure-Grips along with the rear end grease. Some of the new rear end gear lubes already have the limited slip additive installed.

Hope this gives you some help to narrow down the problems . . .