[OrrieG]

The vibration could be the chain. Under acceleration it is tight, let off the gas and the chain gear and shaft are loose and "fighting" the main shaft through the internal spider gears. I could see how that would cause a vibration. You can check the chain by pulling cover #51 in the diagram below. Be careful when you remove it, there are a bunch of needle bearings that support the chain gear shaft. If I remember right I think you can partially pull it and see the chain. If it has an obvious sag or you can move it up and down easily with a screw driver it needs replaced. Your decision then becomes to fix it and put in a part time kit or go with another model of case. FWIW I rebuilt and converted mine to part time using a shaft kit in 1985 and used the truck as my DD, camping and hunting rig until 2000, putting about 8K miles on it every year. This build I looked at the chain it is still relatively tight.