friction modifier

[Wills05xlt4x4]

I'm about to change my diff fluids and was wondering about friction modifier... I'm almost positive I need it since I have tow pkg with 3.73 ls but would I need it for front and rear or just rear? Thanks in advance...

 

[Black Dynomite]

Just the rear.

 

[fusseli]

Just the rear, front diff is open on all models. And ya AFIK the 3.73 LS was part of the factory tow package.