4x4 non-engaging problem

[Guest]

I'm having trouble with the 4x4 on my '97 Expedition. The dash lights go on indicating locking into 4wheel drive. Unfortunatly once on ice and snow, I can tell immediately that I'm not in 4wheel drive. I have crawled under my truck and had someone shift in and out of 4wheel drive and am hearing nothing at the transfer case. Anyone had this problem and what if any solution. I'm thinking it may be the electrical to the motor on the transfer case.

Thanx in advance.

Greg