The keyless entry system on your 2001 Chevrolet Suburban uses "rolling code" technology to prevent someone from recording the signal from your transmitter and then using it to open your truck. If you activate the transmitter 256 times out of range of the receiver, it will be out of synchronization and will no longer unlock the door.
Where is it?
The keyless entry receiver -- called the remote control door lock receiver -- is under the top pad of the dashboard, just to the driver's side of center. To get to it, you'll need to remove the instrument cluster bezel and the upper dashboard panel.