2003 F150 Supercrew 4 wheel drive will not engage in 4hi while moving but works in both 4hi and 4lo while in Neutral. Worked fine Sunday and Monday in the Northern VA snow and ice but Tuesday when I took it to the shop because the emergency brake came apart (unrelated reason), 4hi would not engage while moving. I don't know whether or not it would engage while in Neutral or Park as I didn't try before I left it at the shop. They replaced the 4x4 actuator switch. It still doesn't engage on the fly but does in Park or Neutral.
I feel like it wasn't the switch but I have no way to prove it went into 4x4 while sitting still when I dropped it off as I didn't try.
What would cause the 4 wheel drive to only engage while in Park or Neutral and not 4hi while on the fly?

 

probably the shift fork inside the diff to engage the axles , it has two small plastic tabs on it that ride in the grove of the sliding collar. The tabs break off and no 4x4.