p0721 on my 2003 4.6is :'(
 

my truck for some reason today it decided to throw me a trans fail safe mode p0721. it was driving fine its never given me a single issue untill today. any idea on what could be causing this ? :(

when i go into drive it revs up to 2000rpm and then it jolts forward like if it where slipping then grabbing the gear ?

Here you go! Google search for you.Output speed sensor malfunction, trans failsafe mode - Bimmerfest - BMW Forums

hmmm truck fixed itself today weirdest thing. only thing i did yesterday was i jumped a car that had a flat battery but i doubt that would cause the trans to slip right ?

I had the same issue! My FSU failed so it was draining my battery! by the time i figured it out i had the "fail safe transmission" mode happen to me twice.
when i started the car it barely had enough power to start and it gave me the fail safe sign. i went around the block, restarted the car and the safe mode disappeared, also the slippery throttle went away. same thing happened an other time, by than i knew its battery related. After i changed my FSU, both issues went away.

(hopefully this helps you)