To check your diff. Hold the Spur gear and spin one wheel. If the diffs ok then the other wheel should spin the opposite way to the one your turn. Should be smooth. If it spins the same way then your diffs to tight or needs a rebuild. If the other wheel doesn't spin then the diff would be loose or broke. Also make sure that you put the side with the allen key in it on the drivers side or it will come loose when your driving.
See how you go.
Also when you excellerate and it's not getting drive check that the shaft that goes onto the gear box is spinning, the spur will be but to check the slipper try that.