rock,
I used a combination of need to take care of my family and burnout. I really stressed the scriptural obligation to look after my family first and they didn't argue with me too much. They froze my resignation for about 2 weeks and asked me to reconsider. They finally sent it in but the COBOE was sure that the branch wouldn't accept it. 3 weeks later, a form letter came back confirming my no longer "serving".
It doesn't sound like you have kids and if your wife's health is fine, then you probably can't go with the family route. You can always use depression. You can say that in your current, depressed state, that you feel that you can not be encouraging to the brothers and thus your conscience won't allow you to continue serving in your present capacity.
You could also say that you are burned out and that you really need a break from the weighty responsibility of being an elder.
Obviously, getting into doctrinal issues, saying that you no longer wish to enforce WTS rules, etc isn't a viable option unless you plan on fireworks.
If I remember correctly, you really didn't need to get super specific as to why you want to step aside. Of course, if you are reappointed, then you have to get specific as to why you wanted out in the first place.
PM me if you'd like. I can send you what I sent in.