We don't have synonyms for rereward yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are reward, leeward and rearward.
(n) :
Obsolete spelling of rearward (in the archaic and historical military sense of rearguard and the obsolete sense of haunches, buttocks) . [The part that comes last or is situated in the rear; conclusion, wind-up.]
(n) :
Obsolete spelling of rearward (in the archaic and historical military sense of rearguard and the obsolete sense of haunches, buttocks) . [The part that comes last or is situated in the rear; conclusion, wind-up.]