Clicking Sound

[WayneNC]

 

2005 4x4 silverado extended cab with 40000 mile. I hear a click sound when starting off at slow speeds between first and second(automatic) . smooths as can be on the first shilft ,but amost each time after. If I start faster It seem to shilft smooth. click seems to be coming from rear,but could be slip yoke area. I've read all post on greaseing slip yoke but mine seems a little different. thanks