They are under a big pecan tree and not getting enough sun. They are probably ten to fifteen ft tall.
How to Dig Up and Transplant Mature Crepe Myrtles
Certified GKH Gardening Expert
It would probably be a good idea to wait until fall for transplanting, as this is typically the best time for planting crepe myrtle anyway. In fact, transplanting crepe myrtle while dormant would yield the greatest results. The cooler temps will also make the whole process less stressful on the trees. The following article may be helpful to you when avoiding transplant shock: https://www.gardeningknowhow.com/plant-problems/environmental/learn-how-to-avoid-and-repair-transplant-shock-in-plants.htm