is your  squeak  a   continual  squeal,  or  a momentary  chirp?

I'd think if it's  the springs themselves,  it'd be  a momentary  chirp  as the  spring moves on  acceleration..   Continual  should  be something that is  rotating. 

You'd   get the spring noise  on   a rapid  on off on off with the  gas pedal..  

How's  the carrier bearing situation?   That much axle wrap up   can pound them out  too.  Maybe you have a bearing  spinning inside the rubber mount. 

Can you  find a couple of  buddies  to jump up and down  on the rear end,  really get it bouncing..  if  it's springs,  it'll   tell ya. 

Work bodies  are  rough on  springs.