Just recently, I began feeling this rapid thumping under my driver's side floorboard whenever I let off the gas and coast. If I hit the brakes or gas, it stops. Also, whenever I let off the gas, the truck pulls violently right and when you get on the gas, it pulls slightly left. A sound also appeared in the left rear, that of like a u-joint going bad, but it stills makes the sound in neutral, so it's not a drive shaft problem. Any ideas?

 

Maybe a lose cam bolt on the lower control arm. I'd look at suspension parts floppin around.

 

Does it stop making the sound when you stop the truck? If so +1 on aubrey. But if not i would start diggin deeper. Open the hood make sure your exhaust is hangin in the right places with plenty of clearance. that can bang around too. just do a good insection. wheelbearing?