You can check availability yourself on the Marriott website. Search for dates using MOD as the discount code. When available, they are for 25% off regular rates. Sometimes, you can find a leisure rate that is even cheaper.
It can be very difficult to find availablity at the popular (Hawaii) resorts. Also, it's not a last minute offering. Rather it's just when Marriott has excess inventory and want to offer a discount for Marriott owners.
There are a few Marriott hotels that will also give a MOD rate. The JW Phuket sometimes has a rate, and I don't recall the others.
If you can find it, you're not limited to the Marriott resort that you own. You are eligible for the MOD rate at any MVCI resorts that offer it.