1999 GMC Yukon not crancking

I'd have to be sure the battery has a full charge and the battery connections are good. Next, I'd check the circuits at the starter relay. Terminal 30 is hot all the time? Terminal 86 goes hot with the key in the crank position. There are fuses involved, not only check the fuses, use a voltage tester on the fuse circuit. Even if the fuse is good, if no voltage, the circuit is dead.

Did you try cranking in neutral? There is also a crank fuse, not shown in the picture, that has to do with terminal 86.