Check Engine Light goes on downhill, off uphill

so lately my CEL comes on going down a hill or going down the road but go uphill and it goes off. this makes me think there's a fluid level low. I checked all fluids and all seem fine. the rear diff has a slight leak, if that is low would it make the CEL come on?
Someone mentioned some vehicles have a button to press when doing an oil change that resets the light so it knows when the oil was changed. the idea is the light comes on after 3000-5000 miles to remind you that you need a oil change. does the explorer have anything like that?

ON the first gen, the CEL is not a maintenance reminder light, nor does it really monitor fluid levels. 1st step in determining why the light is coming on is to pull the codes from the computer and see why the computer says the light is coming on. See the stickies in the EEC-IV forum for help in pulling codes.

Did the CEL tests. Key on engine off, 111, 124, 157, 158. and Key on engine on, 412 was it.

Codes are pointing to your Throttle Position sensor and Mass Air Flow meter. Clean the MAF and check the voltage on the TPS. You may need to modify and adjust the TPS to get correct voltage, or it may be shot, it's not too expensive to replace. Hopefully cleaning the MAF is the trick because replacing it is a pricey endeavor.

cleaned the MAF and didn't seem to make too much difference. I was just thinking on replacing the TP. How would one go about checking the voltage?

This thread should answer all of your questions.
http://www.explorerforum.com/forums/showthread.php?t=79362&highlight=TPS+mod