I suspect 2 things based on your comment that when new this never happened assuming "this" refers to the temp gauge rise.
1. the OEM fan clutch on the 05 is "supposed" to be less reliable than the 06 but in any case if you want you can run a quick check by using the method I mentioned (FYI you may also want to get a copy of the 05 manual - the online 05 lacks diesel cooling system data and all system diagnostics but has picture keys while the 06 is the reverse). While there are other possible causes that I mentioned a replacement of the fan clutch with or without adding the nylon fan is a worthwhile investment and may fix your problem per my comment below.
2. the gauge calibration should have nothing to do with this "NEW" problem, assuming the temp sending unit is ok (only way I know to test is pickup an inexpensive IR thermostat from Amazon and check (sometime back geordi posted typical temps at various points on the engine), as you appear to have a real temp rise. BUT if you have the gasser calibration the gauge is going to give you a false high. I suspect you have the gasser calibration because with that calibration you can run all the way to red or nearly so without any of the automatic engine overrides that reduce performance to reduce heat cutting in and you don't mention that happening.
So bottom line my best at a distance quess is you have an over heat problem (hopefully just a dead fan clutch) and a gasser temp gauge calibration. I'd fix the cause of the temp rise before worrying about the calibration. If you do go for the calibation take a copy of the TSB, I can provide if you email me, to make the service writer aware of the issue. In my case I had the gasser calibration and when I went in the

service writer gave me all sorts of static until I said 1) you will do the TSB check because that's all I'm authorizing you to do, 2) if the reader says no calibration was or needed to be done then I know I have a diesel temp gauge calibration and another problem in which case I'll pay no more than 15 min. shop time, 3) but if the reader does the calibration then I'm right and per the TSB I'm not paying

. End of discussion, although I did indicate I'd be happy to have the same conversation with the service manager, and to cut to the chase I needed the calibration and did not have to pay. FYI at least one member likes running with the gasser calibration because it gives him more detailed temp information.
I can say my son's 06 ran in AZ 110F temps without any heat rise regardless of terrain and when we came back from AZ to Tally 3 weeks ago with 2 adults in a fully loaded CRD pulling ca. 2000-2500 lbs in a 5 X 8 UHaul the temp gauge stayed pegged at 1/2 a needle width left of center.