I have an 02'Buick Centry.  A couple of times my brakes on the rear would lock whenever I had to break suddenly.  I would have to put it in to reverse or gear 1,2,3 & then back to drive in order to unlock.  That didn't work this last time.  It looks like it is only my rear driver side.  When it locked this last time I tried to drive in reverse, my rear driver side tire did not move and pushed the gravel but the other rear tire was ok.  What could it possibly be?  

it's time to take it to somebody who works on brakes...

just guessing as it could be several things...

incorrect fluid caused the seals to swell and prevent the fluid from releasing..
(NEVER use fluid from a unlabled bottle in the brake system) just this week i had to tell a customer that they put powersteering fluid in his clutch... a month ago a lady put powersteering fluid into her range rover antilock brakes and drove it for a few days... it locked up also... $6K was the estamate at the shop she took it too..

what else...

worn brake linings...  on some types of brake systems when the linings wore down to the rivets the brakes would lock up...

broken  return springs...  

seized wheel bearings... in places where the car can get into deep water that can happen...

if it has antilock brakes.. there may be a bigger problem with the antilock hydraulics..