2014 Impala rear end noise

My 2014 Impala LT has developed a sliding noise in the rear end when rounding a corner. The noise is duplicted when rounding a corner in the opposite direction. Took to the garage and tech coundn't find anything wrong. Recently with three passengers in rear seat we heared a loud crunching noise when rounding a corner. Took car to another garage. They stated they were able to duplicate problem once on test drive. They checked for tightness on all the bolt in the rear end area and otherwise found no problem. They said the ride was smoth and did'nt suspect rear shocks. Any suggestions.