I had the SOP clutch pack replaced in my 99 supercab at 15K miles for a rumbeling vibration coming from my rear end when I start from a stop while turning. The problem went away for a while but it's back again. First of all what does the SOP clutch pack do and did they replace the correct part for the problem I am having? I saw another thread talking about a similar problem and there were recommendations for changing the differential fluid and adding friction modifier. Any help or recommendations would be greatly appreciated.