TPMS reset?
 

I have a 2007 gmc yukon 2wd. stock eveything... the tpms message came on telling me the back tires were low. they were around 25 psi... filled. then the tpms symbol on dash came on and a message still tells me they are low. they are at 40 psi all 4 tires now.
how can I reset the light without a visit to the dealer??

I was under the impression that the light resets itself (our Lexus system works that way)...often times at work we get "false" tire warning lights if the TPMS switch is in the "2nd" setting, as opposed to "main" (for people who have two sets of wheels/tires for winter or whatever)

Most tire shops, and many auto-repair shops, also have TPMS tools & computers. You could ask your local non-dealer shop to check it for you. You definately do not have to take it to the dealer.

with the key in the acc. mode on your truck simultaneously hold your lock and unlock button on your key fob until you hear a horn. (sometimes this works, other times your going to need a reset tool which your local tire shop should have readily available) starting at your driver front tire let air out until horn honks (this may take 5-10 seconds) if it honks then procede with the same steps going to your passenger front then passenger rear and finally driver rear. if you have done it correctly and the horn has honked after each tire. the horn will honk twice after the driver rear tire has reset. this is to signal is complete. after you are done. simply reset your tire pressure to your preferred setting and you should be good to go.

make sure the truck is off, just in the acc. mode.

Sometimes after adding air to the tires you have to drive a few miles for it to reset. My truck has done that a couple times and my wifes car always does it like that. Just drive it a day and see if the light goes off.