Richard,
Also, don't forget to check the "BIG END" Bushings in your rear leaf springs.
Bad Bushings Cause Clutch Chatter. 
As you can see by the diagnostics people are offering here, most deal with "Float, slack or some sort of Movement" in mounted powertrain or cab. The movement affects & reduces accuracy or precision of your ability to engage a clutch smoothly because something moves out of, or loses, it's adjustment when torque is to applied to the drivetrain system. Clutch moves out of adjustment, clutch grabs, then moves, grabs & moves etc.
If it does it fast enough it creates a terrible chattering effect. The slower the clutch is engaged the worse it gets, and the faster it is engaged the less it is or it may even disappear completely.
If one were to Graphically show what's going on it would be what some of us call " an Inch Worm Effect" or powertrain undulation during torque application. When all is correct & no excess freeplay exists in linkage, drivetrain, mounts or suspension it will work as designed.
By Drivetrain I mean; engine - transmission - rearend/drive axle, related suspension and mounts or linkage used to control use of components, such as a clutch equalizer bar or worn throttle linkage.
I do not know how many times I've made this repair for clients, after they spent huge bucks to R&R a perfectly good clutch & still had the symptoms. Sometimes even worse symptoms because a new clutch has zero wear & grabs better than an old one.
I hope this helps you locate, isolate, and repair your problem.
FBp 
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