Slipping rear end?

Depending on the model of tire, it's not hard to break one loose as you accelerate out of a turn, as one tire is more lightly loaded than the other. Many tires today that advertise 80,000 mile warranties are made of a compound that is so hard, they don't grip worth crap, even when new. Getting new, wider performance tires will help a great deal with traction, as will getting a trac-loc.

Getting a panhard rod won't help much with traction on the street. Those are meant to tame the live-axle instability Mustangs have when you're cornering hard at the track. The only way you'll see an improvement is if you like to take cloverleafs at 60 MPH!

And NEVER CUT OR WELD A COIL SPRING!!! Springs are engineered to work as a complete unit only. If you cut one or more coils off, you'll remove the flat seat at the end of the coil, placing huge stress on the remaining end that will cause the spring to break.
If you cut the spring in the middle, remove a link, and weld it back together, the heat from the welding will destroy the spring's temper. After just a few weeks of driving, the metal will work-harden and fail. Don't risk your life! Do it right and buy lowering springs.