Difference between two tables
FULL joins can be tricky. Try this version:

select
    -- columns from db1 
    a_GUID = a.ObjectGUID,
    a_First = a.first_name, 
    a_Last = a.last_name, 
    a_Full = a.description, 
    a.domain_user_id, 
    a.sAMAccountName, 
    a.employeeID,

    -- columns from db2
    b_GUID = N'{' + b.ADobjectGUID + N'}',  
    b_First = b.GivenName, 
    b_Last = b.sn, 
    b_Full = b.displayName, 
from 
    ( select * 
      from db1.dbo.tb1 as a 
      where a.delete_date is null
    ) as a
  full outer join 
    db2.dbo.tb1 as b
  on 
      a.ObjectGUID = N'{' + b.ADobjectGUID + N'}'
  and a.first_name = b.GivenName
  and a.last_name = b.sn
  and a.description = b.displayName
where 
      a.ObjectGUID is null
   or b.ADobjectGUID is null
  ;