4X4 no more?????

[Tall_guy_ky]

I have a 99 F350 7.3 4 door short bed. 4" turbo back MBRP exhaust, with a superchips programmer, 3 1/2" lift. My son in law liked it so Much he went and bought a 2004 and we have done it the same way only difference is he has a 6.0. His truck has been working fine until tonight, he engaged in 4X4, the light came on but the front end didn't drive. I have climbed under it, and listened for the actuator to engage, it sounded like it went in, I checked the front drive shaft and was no longer able to turn the front drive shaft with it in 4 wheel drive as I could in 2wd. We pulled the hubs and swapped with mine, which worked fine in mine, but still the same. When I drove it, seemed to be driving till under a load (going up an icy hill). Could the actuator be disengaging while under load? Could the sensor on top be telling it to do something different? Please help? The 99 pulls like crazy with his hubs in it? I am at a loss, please help?????
Shane

We have had it lock in and work properly yesterday untill we disengaged it and then back to same issues.
I need help on this?