B757 flight crew encounters severe turbulence east bound at FL370. No injuries occur; however crew elects to divert for maintenance inspection.

Narrative

During cruise; light to moderate chop; light mountain wave; +/- .01 Mach; we sat the passengers and told the Flight Attendants they may need to sit shortly. Approaching approximately 40 mile west of XXXXX; mountain wave became moderate (+/- .02 Mach and IVSI +/-700 FPM) with moderate chop and we sat the Flight Attendants via interphone; followed up with PA of 'Flight Attendants be seated immediately.' Within a very short time; the IVSI swung from +/-1;500 FPM and our .79 Mach increased to near barber pole .84 Mach. The autopilot kicked off during a pitch-up of about 8 degrees; followed by a bank of 15-20 degrees. Pilots took control of the aircraft and requested immediate descent due to the now 'Severe Turbulence.' ATC cleared flight to FL330 and the crew descended in moderate chop. Cabin crew confirmed all passengers were seated and crew was all safe in their seats as well. At FL330 over XXXXX; a few minutes later; a pair of severe jolts was encountered. During this encounter; the Captain was on SELCAL with Dispatch and Maintenance to determine best course of action; i.e.; continue to ZZZ2 or land ZZZ3 where the flight might recover and continue with another aircraft. Both Pilots with coordination from Dispatch and ATC declared an emergency and got a clearance to ZZZ3. The autopilot was reengaged after it was determined the aircraft could be trimmed and flown in a normal manner. An uneventful approach and landing followed with taxi to the gate.