hey sorry to hear that.... but from what i know, the lexus/toyota vehicles uses something called a "floating type differential" on their rear wheel drive vehicles, which means the differential is mounted to the rear subframe via RUBBER bushings, i hate to say it but they all thump under the right conditions, very much like the driveshaft clunk you find on the GX and the LX. changing the bushings will probably mean the rear subframe gets changed and after that it will most definally return within a short period of time. i would still have the dealer double check your driveshaft play and make sure its within spec. and go from there....good luck
I used to have an lx470 and under certain conditions it would do the same thing. It is normal on these models.