OK Now I have a problem! 4-6-6-4 challanger?

It was running great, [V]
Now when I put power to it, it sits and spins
The engine that is. The wheels are
trying to move but just barally it did this
once before I let it set for a while and
it seemed fine, but now I am not so sure.
So I am going to assume its the gear drive
from the motor to the trucks on the gears that
are slipping. How hard is it to get to that area
This engine seems to be a little more difficult
than others I have worked on.

thanks

jef

Who is the manufacture?