Where are the spark plugs located in the 2002 olds bravada?

They run down the center of the engine under the coils. You can see the coils when you remove the resonator (the thing that says Vortec 4200 on it). One bolt holds the coils in place. Remove coils and change plugs.

Actually you will need to remove the two front tire to to get to them and I would advise watching videos on the removal as two have struts blocking direct assess and you will need a flexible shaft extension or an universal joint for those two at least.