scrublover

Check your seat/post/seatclamp.

Then start looking at your suspension pivots/bolts/shock stuff.

Likely just needs a clean and re-grease of things. Make sure things are not too loose/not too tight. Without knowing the particulars of your frame, or its specific torque specs, it's hard to say more. Take the rear shock off and cycle the suspension. If things feel tight or notchy, you may have blown bearings or pivots. Start taking things apart. Clean. Grease a bit. Put back in the same way you took 'em out. Tighten up enough to get rid of lateral play, bolt the shock back up and see how it goes. May need some Loctite or something on some of the bolts to keep things from backing out. NOT on the bearings or pivots or bushings.

You may also just need new DU bushings on the rear shock.

Tough to tell more without actually having the frame to see.