Loss of all power when cold

[Water Dog]

I have a 2000 Ford F-250 with the 7.3 Power Stroke Diesel with 218,000 miles on it. The truck has always been a little gutless for a few minutes after a cold start, and I understand that this is normal, but lately you HAVE to let the truck warm up at idle for probably 10 to 15 minutes before driving or it does not possess enough power to even move itself. It also idles just a little rough when cold. Once it warms up, everything is fine. I have not noticed any smoke. I do not live in a cold climate and the lowest temps we ever see are around 32 early in the A.M. every once in a while. Normal morning temps are in the 40's in winter. In reading some posts on other forums, it seems like it could be related to the exhaust bypass valve, although my symptoms do not match exactly. Any ideas before I dive in and start trying things?