sounds rumble strips when shifting

When my 1999 Ford expedition shift in to top gears at highway speed it sounds like I'm driving on rumble strips when shifting

1999 XLT 4x4 5.4L

Thanks

When my 1999 Ford expedition shift in to top gears at highway speed it sounds like I'm driving on rumble strips when shifting

1999 XLT 4x4 5.4L

Thanks

I had the same problem but only when starting from a stop. Changed my rear diff oil twice with the posi additive and all gone!