Creaking Noises could be a couple things:
1) Check the torque boxes for tearing (if you drive the car really hard)
2) Torn sheet metal where the UCA's attach to the body (remove your back seat to look at this area)
3) Could just be a bad shock
4) Bad bushings (already mentioned)
5) Check your bumper supports for rust (try to move body panels around)
6) Check your trans mount and floor pan areas for other tears...
These are the typical areas where the Fox mustang has problems