they may have set the pinion bearing preload to tight and crushed down the crush sleeve to muc and tried to back it off to lesson the preload, but once the sleeve is crushed it has to be taken back apart and replaced.

That will cause the backlash setting to move around and burn up the gears. Also might not have torqued down the diferental caps to tight. or just hit them with the impact.

A lot of stuff could be wrong, might want to ask for your money back and they replace the R&P, and or find a new shop to do the gear swap.