Sorry for OT but: Are you sure you have diffs wrong way? If they're upside down car would go backwards. Rear diff can go in only one way (there's a cutaway in plastic part on bottom on chassis to make diff easier to slip in) and front diff have to go in according to rear diff because it would turn opposite way (backwards) if inserted wrong. Check in manual how they should go in, there's only one way for them to work properly (go forward).
