Both of my XMODS are messed up!!

[badazzTL]

Now I know the yellow skylines are known to be crappy, but ever since I got it like 4 or 5 months ago it hasn't ran smooth. Something in the rear is messed up and I can never figure out what it is. I've taken the entire car apart about 10 times and each time I have no luck. So I bought the AWD kit and it runs, but it's not nearly as fast as it would be if all 4 wheels were turning @ full speed rather than only the front 2.

THEN my bro has a white supra that he doesn't play with so he gave it to me. It ran fine for a while but then the black part in the rear that connects the wheels to the white gear with the metal pin in it broke so it only pulls from 1 wheel. SOOOOO
I took the AWD kit, put it on the skyline and took the broken part from the wheel of the supra and put it on the skyline. Then I put the one from the skyline onto the supra. Now the supra's engine will turn like crazy and even make the rear wheels turn pretty fast. However, when I put it on the ground it will move EXTREMELY SLOW!!! I took it apart like 6 times already to try and fix it but each time it does the same thing. I'm just about to give up on these things. anyone wanna buy my XMODS for cheap? Sorry for the long post and if anyone can help me I'd appreciate it greatly! They are fun toys but I get aggrevated when I spend more time trying to figure out what's wrong with them than playing with them. thanks guys.