This is the post-cat oxygen sensor (little thing sort-of looks like a spark
plug sticking out of the exhaust pipe, behind the catalytic converter, with
a wiring harness coming out of it), and since the computer thinks the
voltage is high all the time, it could be one of two things (most commonly).
Either it's a melted/shorted wire, as the wiring harness runs right next to
the exhaust, and if any of the hangers/tie-wraps come loose, the wiring
could come in contact with the exhaust.

If the wiring all checks out, then you've most likely got a bad oxygen
sensor.  Replacement is fairly straight-forward...  you disconnect the
electrical connection about 12" back from the sensor itself (the sensor has
a pigtail), and unscrew it.  That's the tricky part.  You either need an
oxygen sensor socket (like a really deep socket, but with a slot cut in it
to allow for the wire pigtail), or a wrench (flare-nut is best, box-end is
second choice, open-end as a last resort).  It's going to be TIGHT.  Use
some penetrating oil on it first (Kroil, PB Blaster, or the like...  WD-40
won't cut it).  On my friend's Durango, the orientation of the O2 sensor was
such that we had to get really creative with our wrench placement, and use
of cheater pipes.

The sensor will run somewhere around $50-$80, and can be had at most auto
parts places.