Most slip joints don't have fittings. You have to pull the drive shaft, if you have a two piece shaft you pull the rear one "CAUTION" if the emergency brake is not set the truck will roll. Better still chock at least two wheels. Make an index mark on drive shaft and rear axle pinion yoke so that you can return shaft to it's original position. Two piece shafts require a index mark on the front shaft also. This might be overkill but if you end up with a drive line vibration it won't be from indexing. I believe GM has a spline grease, resists being washed by the trans fluid or the elements. Good luck with your fix.
I'm having the exact same problem, see my threads under
6 speed Allison. The dealer daid the spline was lubricated OK and
it's in my transmission. Mine doesn't do it when it's cold but gets
worse as it warms up. I see you have a crew cab to. Keep me advised
if you solve the problem. The Allison factory has opened a case on
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