So has any one ever modified a front drieshaft to run on the back, if so how did it work?

it would work like it was a rear driveshaft

if the joints are the same its just the same as a rear. the tubes might be thinner on a front so i would use some heavyer material

oh and make sure you have enough slip in the shaft too.

I ran a CV like the front has in the rear of my rig for a while. It worked fine, but I eventually got rid of it because it wasn't strong enough - I kept breaking the alignment pin. I put a 1350 size joint in it, and have had fewer problems.