On the 99+ 4R70W Transmisson the J-Mod is very easy !
J-Mod is a mod to the Valve Body of the transmission !
Its drilling out six holes in the separator plate and removing two springs in the trans !
It sounds harder then it really is , the worst part is how messy it can be , it was for me I got Trans fluid everywhere !
This is the same info I used to do my Trans , go to page two to get step by step !
J-Mod updates! - MercuryMarauder.net Forums
As for the O/D? , Daily Driven I leave the o/d OFF under 45 , the weakest gear in the 4R70W Trans is 4th , this is because its not realy a gear its a belt , so get your stang up to speed with the stronger gears 1-3 , then put it in O/D , this is so NO load is on the belt getting you stang up to speed , extending the life of your trans , also never Shift from 3 to O/D at full throttle , with the stock gears this is at about 128MPH , very bad for you O/D Belt !
For the Track , off