This is what the CDC protocol on yellow fever vaccination says under "contraindications":
Altered immune states. Infection with yellow fever vaccine virus poses a theoretical risk of encephalitis to patients with immunosuppression in association with acquired immunodeficiency syndrome (AIDS) or other manifestations of human immunodeficiency virus (HIV) infection, leukemia, lymphoma, generalized malignancy, or to those whose immunologic responses are suppressed by corticosteroids, alkylating drugs, antimetabolites, or radiation. Such patients should not be vaccinated. If travel to a yellow fever-infected zone is necessary, patients should be advised of the risk, instructed in methods for avoiding vector mosquitoes, and supplied with vaccination waiver letters by their physicians. Low-dose (10 mg prednisone or equivalent) or short-term (less than 2 weeks) corticosteroid therapy or intra-articular, bursal, or tendon injections with corticosteroids should not be immunosuppressive and constitute no increased hazard to recipients of yellow fever vaccine. Persons who have had previously diagnosed asymptomatic HIV infections and who cannot avoid potential exposure to yellow fever virus should be offered the choice of vaccination. Vaccinees should be monitored for possible adverse effects. Since the vaccination of such persons may be less effective than that for non-HIV-infected persons, their neutralizing antibody response to vaccination may be desired before travel. For such determinations, the appropriate state health department or CDC ((303) 221-6400) may be contacted. Family members of immunosuppressed persons, who themselves have no contraindications, may receive yellow fever vaccine.
"antimetabolites" would cover the 6mp/imuran spectrum. Hence it sounds like they advise against it.