Two things I might look at-
The panhard bar meets the rear axle housing on a pivot joint. Sometimes where that joint meets the axle housing can crack at the weld and only squeak when turning/hitting bumps.
2nd and more probable- Check your rear axle bearings for play. If they're moving at all when you're turning you'll hear a metal to metal from the brakes not being in line.
Just some ideas!
To check the rear axle bearing all I have to do is lift truck up an try to shake tire if it has play then that's bad is that correct??