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we own a 1999 & 2000 jeep grand cherokee & are having lock & lift gate problems. on the 1999 our locks will not unlock the back lift gate, making loading & unloading of large items difficult. lift gate will unlock & open up once in a great while.power locks make kid of a grinding/buzzing noise when button pushed to unlock. in the 2000 the lift gate will not lock. the driver info. panel always reminds you that the lift gate is open. very annoying beeping while driving. we have tried numerous times to close it, it seems to just not click in properly to disingage the sensor.  

I have the same problem.  If you ever figured out what was going on let me know.  

Is there any loose mouldings or screws backed out so door is not closed enough to latch. Had this problem one time with a dishwasher door
which is same thing in principal.
Your buzzing sound sounds like lock is trying to latch but can,t do to missalignment or door not fully closed in line with latch.  

I own a 2000 cherokee and had the same thing happen.  I took the lift gate pannel off to reveal the lock and pully mechanism.  There is a thin wire that is pulled by the latch when the locks are activated to lock or unlock the door.  The wire seems to slide and give enough slack to cause the door to not open.  All you have to do is loosen the screws that are holding the wire and give more tension to the wire.  Test before you put the pannel back on.  Hope it helps.  

the buzzing noise you are hearing is the power door lock , it is part of the door latch,the only repair is to replace the door latch,and the rear liftgate latch needs to be replaced also the sensor is part of the latch,these are common failures,