open the black motor gear cover and see if the spur is rotating nicely when you give some trottle and not buckling or something.
If it passes, check if input shaft is rotating too, some times, even when spur is super tight, it still slips if thr is no clutch pads left.
Next take off transmission skid plate (part no#5637) and check driveshaft and if the output shafts from the tranny is rotating.
Maybe the driveshaft popped off or something. You could check it by removing the plate which is right under the tranny on skid plate held by single screw.
I hope it helps you locate your problem.