rear suspension noise over sharp bumps

[62Daytona]

I have a 1992 SC400 lexus with 162,000 miles.  I've owned it for 16 yrs.   I replaced the rear shocks with HP Blue Tokico and sway bar links.  The rear suspension makes a Thumping noise like something was loose in the trunk over sharp bumps-cracks in concrete.  So then I replaced the 2 lower control arms with new Lexus items on each side.  The noise is still there.  I have the upper control arms but have not installed them.  The uppers will require dropping the subframe a few inches.  I lubed the sway bar bushings-no help.   I used all new bolts/nuts and torqued them.

I wonder if the rear springs are weak?  What about the Strut Rod bushings?  What about the 4 subframe bushings?  What about the 2  Differential bushings?  What about the Driveshaft center bearing mount?

I replaced all the control arms, shocks, sway bar links, tie rod ends and sway bar bushings in the front.  I replaced the Transmission mount.

I have all the maintenance records covering this car.  In 1995 with 30,000 miles the previous owner complained of a thumping noise going over bumps.  Lexus solution was to lube the front and rear sway bar bushings.

I read other Lexus Car forums and it seems that Lexus models emit suspension noises. 

  I'm asking anyone who has had this problem and found a cure.

Edited September 27, 2016 by 62Daytona
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[62Daytona]

I found the Thump in the Trunk .   I found the driver's side Strut Mount has broken.  The shock nut floats up/down.  Passenger side OK.  Temporary fix-a big washer.  Original Strut mounts lasted 25 yrs and 162,000 miles.

Ordered rear strut mounts from Rockauto.com