I am going to get a little wordy here, but will try to describe everything the best I can.

First of all, truck is a 2004 3500 4x4 with the HO cummins and an automatic. 205,000 miles and I just put a new water pump in about 1500 miles ago.

This is my first winter in this truck, and it feels like it takes forever to warm up. Granted it's been -10 or colder here in ND. Anyway when it's been cold it has been taking forever to get to operating temp (25-30 miles+) and if I stop the temp gauge immediately starts dropping. Like if I am warmed up and stop at a stoplight the temp will drop to about 160 or even colder. Anyway this morning I cut a piece of cardboard and made a homemade winter front for the grill (not in between the radiator, actually zip tied to the grill). I just took it for a test drive and it took 25 miles before the temp gauge got to normal op temp with the whole grill blocked out. About 1/2 mile of 15-20 mph, then about 2 1/2 miles of 30-35 mph and the rest was 60mph. It was plugged in all night right up until I started it and I let it idle about 2 minutes before driving. The temp gauge read -8 the whole drive and weather report shows 15 mph winds and "feels like" -27 degrees. The cardboard definitely seemed to help it hold op temp, after stopping it took about 2 minutes of idle before the temp started to drop and it held same temp as in warmer weather (just left of 200) while driving.

So my question, does this sound like a bad thermostat? Or is this just the nature of the beast in sub zero weather?

Please let me know if there's any details I left out and thank you!

 

Based on what you are describing, everything sounds normal. It is not unusual to see engine temp drops if the truck is parked in idle for extended periods. If your t-stat was bad, you most likely would never be able to get the engine up to op temp (185+ F). Most Cummins OEM t-stats are designed to fail wide open to prevent over heating resulting in the engine running cooler than normal.

Here is a link from Cummins that may give you some more guidance.
Winter Engine Operation - Turbo Diesel News -

 

Nature of the beast.

BTW I just had to replace my t-stat on the 03 motor due to overheating. It gradually got worse till one day the dinger let me know it was getting a bit warm @ 240 just cruising. Replaced it and all is well