The only difference in the pro is the IR.. In my eyes the yukon is crap for foxing...
It seems that's where the problem lies .
I know it sounds stupid but did you focus the units properly first? By this I mean with the small lever at the side of the occular lens. If this isn't set to suit your vision, then you'll not get a good image no matter what.
As I said before, the standard ranger should I.D Foxes to 200 yards in decent conditions. The pro can pick them out at 500+ yards (again in ideal conditions). The difference between the two models is that the pro not only has a more powerful Illuminator, but also a more advanced CCD.