DO NOT DRIVE YOUR TRUCK!!!!!
I had this happen and it was when my tranny overheated!!!! Take out the dipstick and smell it, you will be able to tell just by that. Also, you will want to take your truck in and get a tranny flush...that is where they actually pump out ALL the oil and pump fresh stuff through it, then refill. It will extend the time you have on your tranny by a lot.
Write for more info if you have to get your tranny rebuilt, there are several things that I would reccomend having done when it is rebuilt to stablilize the tranny and extend the life dramatically. Also, what all do you do with your truck? Haul? 4x4? Race? What kind of loads do you put it under?
Good Luck