I wouldn't advise it. I have never heard of a factory set-up like that and I'm sure there is a reason for it.
I have owned a vehicle that did have a setup like that, with drums in the front a disk in back. However, it was a Yamaha Moto-4 ATV, not a street driven truck.
I'm in 100% agreement, do the brakes right, with disks on the front to match the rear, as well as the proper master cylinder.
