Forward motion

[T2693]

Took all my spare parts from building my custom E-maxx and rebuilt a stocker setup for my son.

Problem is when I pull the throttle the rear tires move forward but the front tires move backward hence no motion, lol...

What's the fix for this. I thought the diff cup is backwards in the front or is it the drive shaft? Don't want to take it all apart if it's something else. Any ideas?

Thanks