82 SC high speed vibration - rear discshot
 

Was driving this eve on twisty highway and and after 25 min began to feel unsettling vibration under seat, travelling approx 90mph. I pulled over and could smell lightly brake pads and the rear rotors, especially the passenger side were quite hot (spat on em and they sizzled). Front rotors were cool. Open highway so not much braking. Drove slower (around 60 mph) on the way home and no problem. PO upgraded to carrera brakes and otherwise they are good and firm.

Suggestions as what to look for or what this may be?

Sticky piston, stuck pad, degraded flexible brake line, ditry fluid clogging a line, bad MC. These are the first suspects. I would start at the caliper pistons and go upstream from there.
A bad wheel bearing could in theory cause a rubbing disc, but that is far fetched already.

k, thx, had a buddy think maybe siezed parking brake as both rear rotors got hot, will tackle it this eve and see.