to our world!
When I want to know some basis parts info, I like to look at LMCTrucks' on-line catalog. If the part number for a given item is the same for both product lines, or if it says "Chev/GMC", then you know it fits both. It's an easy way to figure out if you can, or if you can't.
As for the GMC grille on a Chevy, it's been done. A '94-98 grille for the pickups will fit your Tahoe w/o issues.
There is/was a company called LateModelRestorationSupply that sold parts for a number of late vehicles, including this body style. They had bowties that you could use on a GMC grille, if that was your preference. I'm giving it some serious consideration myself.