Several things to check... most likely, the bushings at the ends of the anti-sway bar - a repair kit is available from the dealer.

Next, check every steering joint, starting with the pitman arm and follow the linkage and joints all the way to the tie rod ends at the knuckle joint. Have someone work the steering wheel back and forth over center while you look for play in the joints. Odds are you'll find several areas of concern if the joints are original. Check the shocks as well.

As an option, have the mechanic that's doing your timing belt replacement take a look while he has the truck.

Frank