What decides the position of the node of Ranvier?
A combination of differentiation site, chemical guidance during migration, and signaling cues form a variety of sources. 


  The final step in the generation of an oligodendrocyte is the
  development of a mature myelinating phenotype, and this is largely
  regulated by axonal signals. It seems likely that both soluble and
  cell mediated signals from adjacent axons are integrated into the
  developmental profile of oligodendrocyte precursors resulting in cell
  differentiation, up-regulation of myelin gene expression and formation
  of the myelin organelle. Candidates for axonally derived soluble
  factors include FGFs (Bansal et al., 1996 and Qian et al., 1997) and
  thyroid hormone (Barres et al., 1994a and Barres et al., 1994b), while
  axonal cell surface molecules such as L1, MAG, NCAM and N-cadherin may
  regulate formation of the myelin sheath (Payne and Lemmon, 1993 and
  Trapp, 1990).


http://www.ncbi.nlm.nih.gov/pubmed/12385864

I don't know of any diseases that just mess with positioning but there are diseases that kill oligodendrocytes (like multiple sclerosis).