4x4 coupling problem

Hello. I have a 2005 S-10 model (MWM 2.8 engine) and I can't attach the double traction. The cardan that transmits movement to the front differential, couples (ahead) perfectly, but I cannot get to the transfer box, already check the electric motor (it works well) and I do not find the reason why there is no coupling.
I need if someone can give me an expression and reduction box.
Thank you so much