Just make sure its not the hex screw in the dogbone(s). My buggy lost 4wd (lost the front) while I was driving once and it was because my hex screw came loose. If it is make sure you put it in like this:
1. Turn wheel till you can see down the screw hole
2. Turn the other wheel to make it spin inside of it (hard to explain)
3. Turn it till you can see the flat side
4. Screw it with lock tight till it is in there as tight as you can get it

Ok for #2, to get more detail. What I mean is that its like a shaft going threw the middle and thats what you will see turn down inside the screw hole.
#3 The shaft like thing in #2 will have a flat side to it and you will see it if your using a flashlight. Once the flat side is up with the screw hole then you do #4

Hope this helps any and I hope thats the problem.