Yes, if you lowering a truck, you'll need an adjustable panhard bar. Lowering a truck won't cause the rear end to shift out of center any more than a truck at stock height. You just center the rear end at ride height. Actually, with my setup, the panhard bar is close to horizontal at ride height, so the side to side movement will not be much at all.
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Current project: 1967 GMC shortbed/fleetside, 350/700R4, air bag suspension.
Other vehicle of interest: 1957 Chevy Bel-Air 4dr hard top, 283/powerglide, lowered 3-1/2" in front and 2 inches in the rear.
Web Site: jaySpot.com AOL Screen Name: Slammed67GMC Location: Kansas City, MO