When you replaced your wp did you replace or disconnect your temp
sensor wire (brass plug next to thermostat).  Maybe it was loose
before?  My gauge swings between the bottom 1/3rd and 1/2 way point as
I drive when the thermo opens/closes cooling the engine.

This is the test from one of my earlier posts to check the gauge: per
this thread

http://groups-beta.google.com/group/rec.autos.makers.ford.explorer/browse_thread
/thread/81f82700b0d08490/e1241e1de4d26f5f?q=waterskier_erienospam&rnum=1#e1241e1
de4d26f5f

As far as the gauge, it's more likely
the sensor under the hood, rather than the attached gauge in the dash.
The sensor in mine is threaded into the theromstat housing. Looks
like a small brass sparkplug w/ electrical spade lug connection on top
of it. Single plug wire attaches to this lug. I recently replaced my
thermostat,(95 EB) and was able to feel the underside of the little
brass sensor plug in the thermostat housing.
Someone here recomended turning the key on, then removing the wire
going to this brass sensor, then touching to a good ground. The temp
gauge should peg to the max heat position, as the sensor is a variable
resistance type, and you are now giving it full 12volts. IF it does
Peg, it isn't the in dash gage, rather the sensor. I was going to
replace the sensor but after LIGHTLY sanding the underside of it when
doing the thermo, it started working. Probably better to replace the
sensor, as it's about $15 at dealer. Be sure you are getting the
correct sensor though, as there is another one that gives feedback to
the computer for enging temp. Be sure they know what one you want.

There is possibly an air bubble in the system too. Squeeze the top
hradiator hose with the engine cold and radiator cap off and see if you
can work the air out.
Jack