I've always heard it's not wise to have disc in the rear without first or also having front disc.
Can you think of any factory vehicle with front drums and rear disc? Not to say what the manufacturers do is the only way, but what they do is a good guide and pretty much will be safe.
I can only think of 1 off the top of my head, the 1984 Yamaha Moto-4 ATV. It had drums on both front wheels, and a single disk on the rear axle. I had one back in the '80s, almost never used the front brakes, as the rear was sufficient to stop it most of the time.
Of course this really isn't a good comparison to trying it on a full size truck.
