When i spin one wheel, the other wheel spins in opposite direction...so thats normal...but when you try to spin the both wheels...it won't turn!!!

I even took out all the gears and the engine...and it stil won't turn!!!!!

I mean even is there is something wrong inide the diff, then the both tires should at least spin when you roll it, but it won't spin!!

And i have no idea how to take a diff apart...