One suggestion I have is to check the date code of the tires prior to making any decision as if even if the treads are good, if they are 7~8+ years old I wouldn't even worry with using them. However, if they are not old, and you don't ever load your truck to anywhere near its maximum load, then I likely would use them myself. While the math isn't exact here, there is a difference (not a lot) if the spacer is only between the two wheels vs as being between the inner wheels and the axle. Also the width of the spacers matter as well. I would never recommend to anyone that spacers are a great plan however saving the cost of 5 new tires results in a significant cost savings.
So my thoughts are two, what is the age of the tires and do you ever load the rear axle 75% or more of its rated capacity or do you intend to do so? If the age is good and you don't require the truck to support its max load capacity, then I would use them.