Is the sound occurring while applying brakes? I never heard squeaking from my rear brakes, except when I would step on brake pedal on worn out rear pads. Squeaking went away after new pad/rotor change.
If you think sound occurs during rear suspension travel after moderate bumps in the road, it may just be due to outside cold temps... an issue I'm having with my lowered car. Squeaks go away during warmer weather.
2011 Mustang GT Premium 401A Spec- Sterling Gray, aka "Graycie", 6-Speed MT, 3.73, Brembo Brake Package, Current Mods: FRPP-Brakelines/Brake Ducts/Alternator/Radiator/Oil Pan/Oil Cooler/Trans.Scoop/Alum.Axle Cover, Steeda-Sport Springs/3rd Link/Adj.UCA/Alum.LCA/Rel.Brackets/Adj.PHB & Brace/Adj.F&R Swaybars/Adj.Front Endlinks/HD Strut Mounts/X11 Ball Joint/Bumpsteer/CAI, Koni Sport Shocks, ATI Damper, MGW Shifter, Kooks Catback, Wavetrac Diff, Michelin Pilot Super Sport... 93 Octane Performance Tune by VMP