What would cause the tranny to stop grabbing 3rd with NO warning?

My tranny was working fine one moment then the next it just stopped grabbing 3rd with NO warning. It is technically shifting into 3rd but it is slipping like it is in neutral. 1 and 2 work fine so I figure it is my 3,4 clutch pack. Could it be anything else simple to check. I unfortunatly have to drive like this to and from work for a couple of days. I basically just drive to 60 MPH and coast to the next light so I am not maintaining a high RPM. I am considering attempting to rebuild this myself as a can say I am pretty damn mechanically inclined. OR going with the $800 LT1dave rebuild.

 

Most of the time, the 3-4 clutch pack starts to go slowly (due to many things), it is rare to lose it all at once. When the unit is "built correctly", the 3-4 clutch pack "problem", should be a thing of the past.