I hesitate to mention it but as I have found in my life experience it isn't what you say that causes as many problems as what you don't say, so here goes;
if it's a 4x4 then both axles must have the same ratio and that isn't going to happen changing the rear to late model parts. So both front and rear axles must be changed. Look for a 76+ model GM and makes sure the width works with you fenders. 1/2 tons are 3:73 ratio I think.
I could care less if it is a 4x4 just that the truck is already one should I keep it that way? or can I convert it to a 2x4?
