UPDATES!
accompanied my buddy to the workshop this morning and finally found the culprit which causes the rear end to wobble when boost kicks in. apparently, the RH rear lower arm seems to be having extensive free-play at the ball-joint section when shaking the wheel hard. since this section can't be fixed/repaired, will proceed to order new replacement parts (2 x 48710-14110) from Toyota. fyi, the wheel bearings are in tiptop condition as it feels very firm. also spotted the LH differential cushion in poor condition and will order both LH and RH replacement parts (41651-14010 & 41651-14020) as well. do we need a press to remove/install the differential cushion or can it be done on-situ?
thanks to everyone for the awesome response and guidance, it's much appreciated. cheers~