I got 06 Mega cab with 5.9l diesel. My problem when coasting down a hill or flat land the truck feel like someone is pumpimg the brakes. It does lots of chucking, It happens both ways towing the 5th wheel or with no load. Could it be the transmision or just the engine.
thanks to all.

 

I believe it's the transmission. My truck would do the same thing but I installed the Goerend transmission (@ 100k miles) and runs like a champ now. The 2-3 shuttle shift and the downhill bucking are a thing of the past.