Transmission slipping
 

So my tranny started slipping pretty badly while I was going up a hill today. It's on a Chevy 1500 6.5l turbo diesel. And I need to know first off how long I have until it becomes completely undriveable. And how to tell what tranny I have so that I can buy a used one and simply change the fluids and bolt it up and go. I don't have much money right now so this has to be budget. Thanks