2001 Dakota SLT, 4x4

It's developed this little squeak, like something needs oil, when
backing.  It's not the brakes (or brake warning), I think, since the
noise isn't happening when I apply the brakes.  Generally, it squeaks
once, like a door opening (squeeee), then stops.  It may be related to
turning sharply, as I only notice the squeak when backing out of the
carport.  It's coming from the back end, somewhere.

It's not very loud, I only hear it when I've got the window rolled down.
 I've tried the brake setting procedures (driving forward/back and
braking sharply, but no change.

Is this just one of those sounds older trucks pick up over time, or
something that should be investigated?  What might it be?

jmc