Is it possible to change differentials from GU6 to GT4( I think that's pro for 3.73?). I know both front and rear have to be same but just curious if it can be done and if so what other components have to be changed to make the switch.

It's a matter of bolts and nuts. Speedometer, transmission shift points, TCCM, ABS and engine performance will all need to be recalibrated in the ECM.