If I lift my stock 91f-150 8 inches do I need to extend the axles ?

 

you should not have to lengthen the axles. Although driveshafts and brake lines will need to be lengthened. If your wanting to run a tire bigger than a 36 consider a SAS (straight axle swap) also consider re gearing. hope this helps

 

How much do I need to lengthen the driveshaft? Where could I get them?

 

That I am not sure. You may be able to find one from a different truck that will fit. I only lifted mine 4 inches suspension wise and plan on doing a body lift later