The first thing that comes to mind when you said "irritating screech", is mufflers/mid-pipe scraping. How much lower is the car now?
Some of the suspension bolts may be torqued or installed wrong, or you got a bad component, or your making a massive amount of HP and have caused the rear end to twist and get knocked out of place a little.
you should see if there is any rocks in your springs as well. My driveway is a 700 feet gravel mess. My car was making a horrible screeching noise from the front while turning or hitting bumps. I went rotate my tires and found 2-3 rocks lodged under the base of the spring. Took them out and haven't heard that noise since.
1998 GT - CAI, trickflow intake, 3.55 gears, O/R x-pipe, flowmasters, short throw shifter. 145k miles
1995 GT vert - 1 of 36 5.0 Appx. 450,000~ factory motor miles *totaled/parts car* RIP..