Have 2005 Ram 3500, Nv5600 trans, completely rebuilt 100,000 miles ago, started jumping out of 6th gear, when decelerating,  tore down,  replaced 6th gear, syncronizer slider, and filled with gear oil, because recommendation for fluid is too thin, still jumping out of 6th, any recommendations????

Mine will do that occasionally when the exhaust brake is on but not while it is off. Not sure why but has done it for a while. The tranny has 240k on it all with Pennzoil Syncromesh and that is the only issue it has ever had. Sorry I dont know why it is happening. Someone here is bound to know something. 

Slack in the bearings will allow the shaft move forward or backwards and kick the shift ring of gear. 

So, too much and play? So, more shims?

End play, darn spell check 

You'll have to look at the direction the fork is pressed in 6th gear. So if the shaft has enough play it can push the shift collar back out to neutral.