2001 Ford Lariat - 4 wheel drive problems?

A mechanic looked at my truck and told me the actuator solenoid is bad. I can't shift my truck into 4 wheel drive. So, I am trying to find the part so I can buy it. I want to be sure this sounds like the problem. Also, is this a part I could replace (I am mechanically inclined). Thank you in advance for all your help.