The undertaker is not an overdrive, but an underdrive, ie a reduction box that INCREASES your RPM's if the R&P's are left the same as you currently have installed.
The main useage is when this reduction drive is installed along with lower diff gears(4.6) to gear for 33"+ tires.
You will need custom driveshafts front and rear as well, and I would seriously consider slipjoint eliminator output shafts.
On a 2dr it will really shorten the rear driveshaft, the best application is with the longer 4dr sidekicks