Where is the egr valve located for a 1998 Chevy Tahoe?

The EGR valve location on a 1998 Chevy Tahoe (assuming a 5.7L V8 engine, as that was the most common) is on the intake manifold. It's typically located on the passenger side of the engine, near the back.

However, pinpointing the exact location without seeing the engine bay is difficult. It's a relatively small valve with a vacuum line connected to it. A diagram or a picture of your engine from a repair manual specific to your vehicle would give you the best visual identification. You can usually find these online or at an auto parts store.