When my tranny shifts from 3rd to 2nd gear in my 2006 Silverado a clunking sound comes from the rear end, any ideas as to what it is?

 

I don't know if u already figured it out but I have a 2005 silverado and I have the same problem. This is very common. U have to grease the yoke. It is simple to do. There four bolts that hold it in. Carefully remove the bolts and pull it out. Grease it and put it back in. This is only a temporary fix but it works. U just have to do it every couple months