Which xylene are you working with? From your description of your observed products, I'd guess o
-xylene (1,2-dimethylbenzene), but just wanted to be sure.
Do you really need to hit it as hard as fuming nitric? Given the methyl groups would activate the ring, perhaps nitronium tetrafluoroborate, or similar?
Have you seen Proc. Chem. Soc., 1961, 357 http://dx.doi.org/10.1039/PS9610000357
From the googled cache it seems that the 4-isomer predominates here ~2:1 over the 3, but I don't have access to the full paper to be certain.
Or perhaps David A. Burgess and Ian D. Rae, Aust. J Chem., 1977, 30, 927-31 or Lennart Florvall and Maj-Liz Persson, Acta. Chemica. Scandinavica B, 36 (1982) 141-146 - both report use of 4-nitroxylene in further transformations (from http://www.patentstorm.us/patents/6132944-description.html
What do you need it for, perhaps another way around the maze might help?