I've read a lot of reviews about the Detroit TruTrac...... Can anyone tell me if i have to buy the differential AND also a set of gears...?? I want a 3.90 ratio...... I've read on most sites who are selling the Differential that say "works with all gear ratios". So does this mean I need to buy the differential and a set of 3.90 gears and take it to a good driveline shop for the install?? I need to find a good shop in northern florida....