So, today I had the mustang up on jacks, and when I took it off, and the shocks compressed back to normal height, it started to hiss... I've never heard that before. Now it squeaks on every bump... to me, sounds like I need to replace the shocks... what do you think?

 

The hissing noise was probably gas or fluid going through one of the shocks valves. If you think the shocks are bad, remove one and try compressing/extending it by hand. If it does so easily, it's more than likely time for new ones.

Dave