So the other day I noticed that the car was a little sluggish. After accelerated a bit harder on the expressway I noticed the rear brakes freed up but felt sluggish again going back to city driving. Are the rears seized? Is there a fix? Do I 100% need new calipers?

 

It's likely not the caliper piston, but rather the slide pins on the torque member.

Use a vise to get them out then a bench grinder to clean em up.

Also, this is not "advanced" braking.