n8sf150

I am still having problems finding out if I can change my generic electrical module or (GEM) on my own or if a dealer has to do it. My problem is I pulled a trailer one weekend and now my four wheel drive will not work.I have no power to the switch or the motor at the transfercase.I checked all fuses and relays and are ok.A ford tech told me ther are relays way up under the dash (cant find any) that could be the problem.I dont want to spend $95.00 just to here them tell me what I already know.My truck is a 2003 f150 super crew fx4. If any one has any ideas or a web site that might help it sure would be appreciated. thanx N8