So my 4hi and 4lo lights work and my vacuum actuator works good too. They both become solid fairly quickly actually, but seemingly no front wheel spin.
I don't hear any grinding or bad noises even when engaging 4hi at different speeds. But I seemingly have no front wheel spin, as I got stuck in the mud fairly easily the other day and I didn't see much mud on the wheel compared to the one in the rear that was obviously spinning.
Any ideas? I don't think the 4x4 lights would turn on if the system in the front diff wasn't actuated? Hmm.

Cheers.

 

When you say the vacuum actuator works are you saying that you can see that it is being actuated when vacuum is applied? If so, that would exclude a vacuum leak.

Could the cable from the vacuum actuator to the front axle have become disconnected?

I think you hinted at knowing that there is an engagement gear on the front axle that engages the front axle by the vacuum diaphragm by your comment of not hearing any grinding noises. I don't think the lack of grinding noises is necessarily an indication that the engagement mechanism might be bad.