I just installed a set of used 20" wheels with 305/50/R20 Nitto Terra tires. Immediately after installing them I noticed a vibration at braking. It only happens at about 5 mph, and is more noticeable around corners. When I had my stock 17 inch wheels on it would vibrate if I made a sharp corner. Did Ford use Friction Modifier when these came from the factory. truck has 37k on it. Would a fluid change in the rear end help. What fluid do I need for this year truck. It is a 4x4 with a 5.4 liter.

 

New specs for the rear is 75w140 and if limited slip, use friction modifier.

 

Check those new wheels and tires for balance, belt separations, and out of round, and use a torque wrench on the lug nuts.

 

I had one of the mechanics at my dealer where I work change my fluid. We used friction modifier, and 75/140 synthetic oil. only been able to drive it about 5 miles so far, but it still vibrates at about 5 mph, and when braking. no cornering vibrations anymore though I can't see how an out of balance wheel would vibrate at 5 mph, but crazy stuff has happened.