I don't know about the 1/2 Ton, but the 1 Ton has a backset (not offset) of around 3". You can't put a 4" backset on a 1 Ton.
There are really huge tires that you can get for 15" rims, but they are really wide, and they require wide rims. The offroad people use them.
They also sell a 255/85R16 tire which gives you an even larger diameter. I think that I will use them on the back of my 1 Ton. It is a flatbed, that is with no rear fenders.
