Crossfiring can be a lazy bad habit that is hard to break. (crossfiring, by the way, is when the front end is in one lead and the hind end is in the other lead -- usually the outside lead)
I, personally, never let a horse crossfire just so they will not get comfortable with it. I always get after them hard and chase them around harder and harder until they bring that inside hind foot up and get both ends in the inside lead.
I never let them slow down to get correct -- I always make them go forward faster until they get it right. I do the same thing under saddle. They will get sloppyer and sloppyer and slow down and break stride if you do it any other way. Every horse I have will catch the inside hind leg back up if I just yell at them because I have tried to eat them up whenever they let that hind end fall out.
Letting a horse drag a back lead is kind of the epitome of anti-collection. If you have let a horse do it for a long time, I guarantee that horse does not want any part of collection and of properly wanting to use its hind end.
At this point, I would bit this horse up with a snaffle and a surscingle and force it into proper frame and start longeing all over again -- both on a line and in a round pen -- and teach this horse to properly use its hind end.