Do you know if it is the original third member or maybe a later one? The reason I'm asking is the later model "489" (the last three numbers on the casting number under the drivers side of the pinion gear ) third members had crush sleeves on the pinion gear to set the pinon bearings preload and they can do cause problems with the pinon gear running hot if they are not set up correctly
Does the thump only occur going around corners while accelerating? If so maybe it is the posi causing that