This trial is testing a new drug, pacritinib, to see if it is better than the current standard of care for treating patients with primary myelofibrosis, post-polycythemia vera myelofibrosis, or post-essential thrombocythemia myelofibrosis who have severe thrombocytopenia (low platelet count). 399 patients will be enrolled and randomly assigned to either pacritinib or the current standard of care. The primary outcome measure is spleen response rate.