You don't have to change it...hell you don't even have do the plenum if you don't mind adding oil and all the other issues.
Theres a quick way to determine if you should. Crawl under the truck, grasp the crank pulley/balancer with both hands and try to rotate it. If you feel a "clunk" or play (thats the chain slack against the valvetrain) and you can rotate more that an 1/8 inch to get that, you should replace it. As stated it can only get worse.
Worse meaning it will throw off your PCM because your cam sensor and crank sensor are not in sync, among other timing issues. At worst, it could break or come off when all the nylon teeth on the cam gear disintegrate. The carnage at that moment can vary, but running a valve into a piston isn't much good on either component.
So why not check it, change it, and replace a couple of potential high mileage leaks as well. Just sayin...
Last edited by dsertdog56; 11-20-2009 at 11:27 AM.