My 2018 3500 dually 4x4 has an issue with slack in drive train. sitting still and you take off normal there is a short “slack“ period before everything feels tight and normal. No clunking, no noise, but you feel the “bump” when this happens. When driving and you let off the pedal and go back on it you feel the slack again. It has done this since I bought it a year ago. Funny thing is yesterday in traffic I flipped it in to 4WD and the drive train is tight like a new one no slack at all. Everything under the truck is tight and can find nothing visible wrong, no leaks, all bolted tight, drive shaft solid in park. Thanks in advance.