I don't know of any definite discounts offered. However the Florida AAA travel guide has several companies listed: www.Homes4uu.com www.prestigevacationhomes.com www.holidayhomesorlando.com
I've never used any of them, so I don't have a recommendation for any of them. Also, not sure if they give AAA discounts, but since they're listed in the AAA book it doesn't hurt to ask.
When we rented in 2003 we used orlandovillas.com.
I'm not sure of any military discounts but I'm sure someone else will know.
You might get a better deal going directly to the homeowner and asking for a discount. The best deal I ever had was a user of this board who has a Townhouse at Emerald Island www.orlando-rental-home.com High season should have been nearly $900 for the week and got it for $600.
We stayed at a nice place called www.disneyuniversalcondo.com and got it for less than the advertised price. The owner was very nice and I bet he would give a military discount.