Drivetrain problems

My brother its the owner of a 03.5 msp, it has around 150k on it. We just replaced the motor with a 01 fsde at 145k. WE installed a new clutch, loctited the flywheel bolts ands torqued them to the factory specs. We torqued the pressure plate bolts to the factory specs and filled the trans with 3 quarts of gm syncromesh. The car drive fine up until today. You can baby the car up to 1500 rpms and nothing happens but ast soon as you go over that or put instant torque on the trans, you get this grinding sound and feeling. We put the car in jackstands and it drives through the gears fine.
Not sure what it is but if you have experienced this problem or have any suggestions, please help.
 
Most likely a broken differential, there are many many MANY recorded failures on this site regarding broken limited-slip diffs
 
Back