The stock gauge is a piece of crap, but its there.
You should have an electronic monitor on that engine just to watch the EOT (engine oil temp) and ECT (Engine Oil Temp) if no other parameter. Those are the two vital readings you should know. That electronic gauge will pay for itself the very first time you save yourself from the typical EGR/oil cooler failure.
Scangauge2 is fairly inexpensive ($165), its even available at Autozone.
The turbo vanes can stick from a build up of soot in the exhaust side. That is where the vanes are. If the wife is not using cetane boost in every tank of fuel and lets it idle for extended periods of time, it could be the "sticking vanes" issue.
Clean the EGR valve and take a peek in the hole. Even better would be to park the truck on an incline (nose down), pull the egr valve and clean it but don't reinstall it, cover the hole and let it sit like that for 5 or 6 hours. After that period of time has passed, look for any moisture inside the intake hole. That would point to a leaky EGR cooler. The EGR cooler failure would send coolant into the exhaust side of the turbo and mix with the soot. That gunk will make the vanes sticky. You may not even notice it if the EGR cooler has failed and just hasn't failed to the point of GUSHING coolant.
I don't want to freak you out, but there is the chance that this is the reason.
There are also a few other things you can check/clean that are monitored by the PCM to control the turbo.
First is the MAP sensor hose/nipple. The rubber hose that comes off the intake and goes towards the passenger side of the truck (to the top of the HVAC box to be exact) is the MAP sensor hose. Remove the hose from both the sensor (on top of the HVAC box) and from the intake. Check that hose for any pinholes or cracks that may be there from age. Replace if you find anything. Check to see if the hose is clear. Also, check the nipple on the intake. Poke something down into the nipple (that can not drop down into the intake) to make sure that it is open.
Next is the EP sensor. 05 Excursion has the 04 engine I believe. There are two locations that this sensor is located depending on the year. I THINK
your EP sensor is on a bracket at the thermostat housing. Its a sensor that sits on top of a stainless steel tube (about the size of a brake line). Use two wrenches to remove the sensor from the tube (one on the tube, one on the sensor). You just want to brush the crud off the sensor, but you want to make sure that the tube is not plugged. Use tiewire and feed it down in the tube. That tube just goes to the exhaust manifold on the drivers side so there is nothing in there that your going to break by pushing a piece of wire down into it. If you can't locate any tie wire, you might be able to use brake cleaner.
If you do not see that sensor at the thermostat housing, it will be on the drivers side, under the degas bottle on the front FICM bracket at the valve cover.
If you clean and check the EGR valve, MAP hose/nipple and the EP sensor and ther is no change...post back.
EGR valve cleaning link
If your interested in tackling turbo cleaning. You should purchase the turbo hardware kit (a whopping 8 bucks). It has the o-rings for the drain tube and the gasket for the oil supply line and new turbo mounting bolts. You can also get a kit for "turbo reconditioning."
Heres a link to turbo cleaning
As an afterthought to that long post above: You are using Motorcraft filters, right? That is a requirement and not a suggestion.
It is suggested that you run a full synthetic oil versus the "dino" oils. Long explanations for both.....not gonna type it all out if you already know this.