As title states, my rear brakes are occasionally dragging and 'bumping' as I drive. They drag a lot more on wet pavement or when the car is still cold, but still drag a bit when warmed up and dry.

I pulled the wheel off and checked the shoes and drums, but they look brand new (seriously, I bought new shoes and drums but they look nearly identical). And there are no grooves in the drums or anything...

Any ideas on what's causing the dragging?

the front brake lines rot on the inside and clog up keeping the brake fluid from reversing when the pedal is released.it may be a bad rear hose.

Is it both back brakes or just one side? If it is both I would agree with zenish. If it is just one side it could be a faulty wheel cylinder.

Both occasionally, but the right side more often then the left.

I would try readjusting them before anything else, probably just too tight from the sound of it, think about it, worse when wet and cold, in the wet water can build up on shoe pad, and drum, creating an interference layer if you will, and when cold things shrink, once you drive in the cold, or in the morning when dew is out-does it get better after driving for a little bit? The info given says adjustment to me.

I'm not sure if you've resolved this, but I'd double check and make sure you did not break (or maybe improperly installed) brake shoe retainer spring...that will do it too. You should also make sire you don't have the park brake cable too tight.

your wheel cylender is leaking while it sits. thats why its worse when its cold. have them replaced.