Attention to detail my friend, attention to detail.
Not trying to toot my horn here, but I have a knack for resolving, and troubleshooting. I have to do it all day long. lol
I think a new set of rear springs are in order for you, good sir. Glad we could figure out the issue, even if it was so far along after the initial discussion!
Also, I hope this will help others who come into this situation. Mixing a larger spring, with rear suspension mods, may be asking for trouble.
VISIT FOR PRODUCT ADVICE, TECH AND SUPPORT ON FACEBOOK. www.facebook.com/BMRMustangSupport