'89 F350 Dually. On decel, foot all the way of the gas, I get a vibration through the floor and a howling noise. This just started during the week. It doesn't do it on accel, or sitting in park or neutral, just when moving. Any idea what may be the cause? 460, C6, US Gear Dual Range, D80. Opened her up and this is what I see:

 

Have you checked your U-Joints and yokes? Under acceleration or under load everything is tight. As soon as you let off the throttle the driveline relaxes a bit and you could get your vibration and whirring.

 

Just pulled the drive shaft. Pinion yoke was loose. Pinion nut was finger tight. I'll tighten her up for now and start calling shops on Tuesday to see what it will cost to fix.