i got a 2000 gmc sierra 4x4 on 35s intake exhaust superchip the trans blew out 3rd and od i was just wonderin what i need to fix it myself an what caused it

 

With what you have described it is the 3-4 clutch pack that has failed. It is a pretty common failure so the cause is just what it is. I would suggest having your transmission gone through or replace it with another well built unit. Depending on the application and what your goals are with this vehicle will determine how built it needs to be or if you should upgrade to an even stronger transmission. Vince