I havent checked to see if it does it in 4x4 with hubs locked...

but while either hubs locked 2hi or 4hi hubs unlocked, there is this crazy vibration that starts around 20mph...

The couple times that this has happened i slowed down till almost a crawl and it went away then I accelerated and it didnt come back...

I havent played around with it too much and the only time I have really used 4x4 is at near crawl speeds...


Where do I start looking?

86 reg cab manual

 

I had something similar on my '94. I rebuilt the side with the vibration, including new wheel bearings and grease and the problem went away.