You could move it now as long as you and your friends won't get bad backs from doing the move. Try to take as big a root ball as possible and use root stimulator and transplant shock solution on it in additon to proper soil/hole and mulch. I find it doesn't matter what time of year I transplant/plant crepe myrtles and they do well as long as they are well watered.

With a tree that size, I would really recommend that you wait until the dormant season. It is much less stressful on a deciduous plant, as you know. You could, however, begin some root pruning now, in preparation for this transplanting project. I would do everything possible over the upcoming months to help your plant grow a heavy feeder root system close to the trunk. Applying a layer of mulch and root pruning will assist in accomplishing that.

The very act of root pruning is invasive, so have your mulch layer down first, and be prepared to water your plant heavily at first. Baby it over the summer, but do NOT fertilize!

Read up on transplanting techniques. Improper handling of the roots and the rough 'manhandling' of plants can cause permanent damage. It's easy to do it right, though!

Here is a link that might be useful: Check this out....

A neighbor gave us a large crape myrtle from his yard, and I was amazed at how well it did. I don't recall what time of year we moved it, but the next season it was fine, with plenty of blooms. I know lots of other shrubs and trees that would not have survived that move, they must be tough.

I have moved several large trees, hacked at the roots til I was tired, then pulled them out with my truck and a rope, they are all doing fine!!! Moved mine in the spring. One fell over from the weight in a rainstorm before it got settled back in, but I propped it back up and its blooming and happy later the same year...cm's are tough!