Old55

I just bought a new drive belt for my Craftsman 1000, 5 years old. After I dropped the deck and cleaned that, I was on my back staring at the drive belt and system. Dirt was caked into crevices (and there are a lot of them) near the transmission. I used a bamboo skewer for shish kabobs to poke around in there and accumulated quite a pile of dirt. Checking the belt, it didn't seem like it needed replacing. After reattaching the deck, the test drive showed that the caked on dirt was not allowing parts to move where they needed to move. Check that first before doing anything. I don't think you'll really need to even drop the deck.
One more thing... I will now use a leafblower to move the dust and dirt off the mower; no more water that cakes on and compacts the dirt.
Hope this helps because it's sickening to think about dropping hundreds on a new tranny.