This will be another work in progress. Diurnal temperature range (DTR) should decrease with water vapor feedback, but there are several things that complicate using DTR for proving or disproving anything. The first is that the change is small and the accuracy of the data likely not all that great. Siting issues that may impact surface station readings would be as much or more a problem with the DTR. So far, the Tmin data provided by the Berkely Earth Surface Temperature (BEST) project looks pretty good, but the further back in time you go there are major issues and not using anomalies, that would be amplified.
That 1985 shift is likely due to the southern ocean shift that can be seen in the GISS LOTI regional data.
So as I wade through the BEST data sans R programming, I will try to flesh this out a little more. However, that will not be a big priority since I am sure this stuff must have attracted some attention elsewhere.