I own a 2006 Dodge Ram 2500 Mega Cab with 5.9 cummins (automatic) with 90000 miles. About a month ago I started hearing a single clunk noise when I was slowing down and braking. It would only happen when the truck got down to 19mph. If I turn the overdrive off it does not make the clunk noise. If I turn the tow haul mode on it does not make the noice. If I shift to neutral and then brake it does not make the noise. Only seem to occur when the truck is in overdrive. The truck accelerates and shifts smoothly.

I took the truck into a local mechanic and he checked the rear dif, ujoints, brakes and could not figure it out.

Anyone have any idea what this is or have the similiar problem?

Thanks for the help.