Selecting a group from groups based on one row condition
I assume you want all rows of the group(s) where at least one row of the group has sLossCode = 'REVSTD'. In this case, you don't need GROUP BY. You can use either a self join (but that would only work under certain UNIQUE constraint requirements) or an EXISTS subquery:

SELECT s.sContractNumber, s.sLossCode, s.iServiceOdom
FROM service_table AS s
WHERE EXISTS
      ( SELECT 1
        FROM service_table AS q
        WHERE q.sLossCode = 'REVSTD'
          AND q.sContractNumber = s.sContractNumber
          AND q.iServiceOdom = s.iServiceOdom
      ) ;