Trans & Diff fluid change = CEL... Why?
 

Switched over to Red Line mt-90 and 75w90 this weekend. Also replaced the tires at the same time. Left the dealership with a check engine light. I'm guessing it's due to the fluid level drop in the tranny. Does the car know when that fluid is low? I'm pretty sure that's all it is. See anything wrong with just resetting my ECU?

Why don't you get your CEL code pulled and remove all the guess work? The CanScan and Scanalyser tools are very good investments. Might be something as simple as a loose gas cap. There's no reason changing your fluids should trigger a CEL unless your dealership screwed something else up (which is another great reason to this this easy work yourself).