If the slip yoke is pulled apart (drive shaft removed), cleaned & regreased properly this will stop the "clunk" instantly. Either that dealer did it the cheap way by greasing the SY with out pulling the drive shaft (just slid the boot back, rubbed some grease on the exposed splines, put the boot back & installed new clamps) or they missed something else- such as a bad universal joint. Either way I would get it back to that dealer ASAP & explain that the regrease didnt work. G/luck
Joel