The front driveshaft engages but the hubs do not lock. Any ideas out there

 

Some more information about what your truck has on it would be helpful.

I'm going to assume that your T-100 has the A.D.D. system on it, since most that I ever looked at did. If that is the case, I would start looking at the vacuum hoses that run the mechanism. If any of the hoses are bad, or disconnected, then the A.D.D. will not operate properly, if at all.

It also occurs to me to ask how you are sure that the front wheels are not engaging. If you are relying on the dash indicator light, you might be mechanically in 4WD, but the light isn't working, because the wire running it has become disconnected along the passenger side of the engine.