If you've got the $$, JL Audio makes custom enclosures that go under the seat of the 08 crew cab, but they're pretty pricey. I know my 05 has enough room to put a couple 10s behind the seat, but I'm not sure if the 08 is different as far as cab room goes. I've only had my truck for a couple weeks, so I haven't gotten to that point yet. I do most of my research on Crutchfield.com. Most of the time it will tell you what fits your particular truck. Hope that helps a little. Good luck!