If my grandmother financed a truck for me and recently died, ihow do I continue to pay for it?

In general, the finance company has the right to demand return of the truck or full payment on the loan after your grandmother's death.  In addition, if an estate is opened on her behalf, the personal representative must notify the finance company about her death.

If you can possibly transfer the loan into your name, I suggest you contact the finance company, explain the situation, and get the loan transferred.  The finance company probably does not want the truck back, but if your credit is truly poor, they may demand it.  If you have been paying the loan and no one opens an estate for your grandmother, the finance company may not learn that she has died.  Of course, if the truck is in your grandmother's name, you will have to have it transferred into your name. 

I suggest you consult an estate lawyer in your area for advice.  Achieving peace of mind is well worth the cost of a consultation.