I am looking at buying a newer CTD, and noticed that the 1997 reg cab has a Standard duty 5 speed Tranny, and the club cab or dually had a heavy duty 5 speed tranny. What is the difference?

As far as I know, all the Diesel engine option trucks get the NV4500HD...

I believe the only trucks to get the standard duty tranny is the V8 gassers.

some had different transfercases as the 5spds came with either the 241LD without a pto cover or the 241HD version with the PTO cover

the 5spds are all the same in terms of output splines and guts the top cover changed though