Newbie question about rear diff / drive shaft

Hi!

I have an RC8.2e RTR. I recently replaced my rear diff and I am pretty sure it is working fine (when I spin one wheel, the other one spins the other way etc.).

I reassembled everything on my pit stand, connected the main drive shaft from the center diff to the rear diff, hit the accelerator, and the wheels started spinning like normal. However, when I put the car down on the ground and hit the accelerator, it just make a loud buzzing noise and doesn’t move. I can’t figure out what...

Newbie question about rear diff / drive shaft