Sounds like the fairly common Camry clunk - Check the rear stabilizer/sway bar bushings. They are about a foot in from the tires on either side, should solve your issue, very cheap.
'93 camry, '96 camry, '99 camry, same noise/fix.
Reference:
http://turboninjas.com/camry/sa.pdf page 95, look for "bushing"
Edit:
You can check these easily enough...give the shake test, if you can move the bar inside the bushing, they're worn - at 138k, they're likely worn.