If a rear diff is replaced with a different size, will the front end have to be replaced? ( in a 4x4)

 

As posted above by cumminalong yes you have to change both diffs, if you do not then when you engage 4 wheel drive bad thing will happen.

 

The ONLY time you see guys running different ratios is generally mud racers. They will use the same size tires, but will run ratios that are, numerically, one sizelower in the front. This gets the front wheels tuning faster than the rears and keeps them going strait.

The only other time is when you see guys with stuff like sand drags that have different size front and rear tires. Then they use different gears to off-set the difference in tire size.

But on your truck....YES, you MUST change both sets. Otherwise.......a little bit of traction leads to binding, which leads to crash, boom, pow! Driveline damage. []