The driven gear is on the end of your speedometer cable where it goes into the transmission. You'll spend more time jacking up the car than you will swapping out a speedo gear!
Speedo cable-to-transmission hold down clamp is held with one 3/8 bolt. Remove bolt and clamp. Pull out speedo cable -- you may have to wiggle it a bit. The driven gear is held in place on the end of the cable with a spring clip. Remove spring clip and slide gear off cable. Reassemble in reverse order. Simple, Eh? That's all there is to it!
You might want to replace the o-ring on the end of the speedo cable just to avoid any leakage of tranny fluid, but this really isn't necessary.