Quaking aspen suckers are just difficult to transplant. Expect a much higher percentage of casualties than normal. I've seen a lot of natural generation and regeneration of quaking aspen in Minnesota, Wisconsin and South Dakota, and they NEVER sprout every 18 inches. Perhaps they do in Utah, though.
Regeneration after a fire or logging (the most dense proliferation) is more like every 4ft or so. And of course, even all of these would never grow to maturity. In the end, you will want your trees to be 10ft apart or more. I would figure on planting two or three times as many suckers as you want trees.
An single clone colony of Quaking aspen in a Minnesota prairie.