[DW]

Caleb; I wouldn't worry about it. Here's what you can do to ease your mind: (1) Look on the ATF dipstick & see what type of fluid is used; (4) buy 4 qts. of that type of fluid--if it's Dexron III, any name-brand of fluid is fine; (3) when you next drain your engine oil, also do an ATF pan drain and put the 4 qts. of ATF in through the ATF dipstick tube. That'll ease your mind and if you'll do that every, say 15,000 miles or so, you really won't need an in-line filter. Sure, there were some early Tundra pickups with problem transmissions, but not that many. I'll just bet that those with faulty planetaries would have crapped out if they'd had the best in-line filter in the world and fluid changed every time the motor oil was changed.
If you'll buy a Tundra-specific manual published by Chiltons and others (they have pictures) and do the ATF and other simple mantenance yourself, you'll know for sure your pickup is being maintained right and will not feel (justifiably or otherwise) that some 'snake-oil salesman' either ripped you off financially on your maintainence, or--worse--harmed your vehicle.
The chances are very, very good that you have a good, solid pickup and that your transmission is okay and that nothing that's been done to it since you've owned it has hurt it. These Tundras are fundamentally pretty dog-gone good rigs.