Hi All,
maybe this can help to at least know where to check for tail drop counters:
http://kb.juniper.net/InfoCenter/index?page=content&id=KB20666&actp=search
Basically tail drops are checked using:
Switch@juniper> show snmp mib walk jnxCosIfqTailDropPkts
Still unclear on what the ifOutDiscards really matches.... if someone has found this please share 
Hope it helps.
Lau