On my 4x4 f150 2003 went i put it in 4 wheel low or high with maunel shifter on snow back wheel will spin but front doesnt .only used 4 wheel manybe 10 times ''

You'll need to check the vacuum lines. On the passenger side firewall under the hood, there are two electrically actuated vacuum solenoids. The GEM grounds one or the other, never both at the same time. What commonly happens is that the disengage solenoid sticks, so when the engage solenoid comes on, you end up with vacuum on both sides of the actuator in the center-axle disconnect, and it fails to engage. It's possible that yours isn't fully disconnecting the disengage side when it should. You should be able to remove the two lines and check for vacuum on both of those solenoids (one or the other, never both simultaneously) as you switch from 2wd to 4wd and back. A crack in the vacuum line, or a bad Center-Axle Disconnect could cause the same issues. It's a little tough to troubleshoot remotely, but that's where I'd start.

Start with the vacuum solenoids. If those seem to be working properly, check the other end of the lines where they connect to the disconnect. (If the vacuum is good on both ends, there is no need to trace the lines to look for leaks, but it's not a bad idea) If the vacuum system works, you may need to disassemble/replace the CAD and/or check the axle for some internal damage to the unit. That's even tougher to troubleshoot and remotely.

The vacuum actuates the center-axle disconnect on the front axle. Basically, in 2wd, it breaks the passenger side axle shaft between the diff and the halfshaft. That allows the differential carrier and front driveshaft to stop turning when in 2wd. The spider gears in the front diff are still driven by the driver's side halfshaft, but as long as the fluid level is good, they don't pose much of a drag on the system.

The center-axle disconnect has a vacuum diaphragm and actuator that slides a collar over the two halves of the axle shaft. I suspect that either the actuator is sticking (in which case it probably wouldn't engage in the first place) or it may not be fully engaging (they sometimes get rusty or gummy and only engage part-way). It's also possible that the fork itself is bent or broken, or the nylon feet that engage the collar have worn enough to allow the gear to disengage under extreme loads.

I would do one of two things: Take it to a good driveline shop and have them drop the front axle to see what's going on, or hope a dealership doesn't rake me over the coals on the diagnosis and repair. The only other option would be to disassemble the CAD (If it can be done without dropping the axle... I don't know offhand). If you're dropping the axle, I'd pop the cover and see what you can see in the front axle too. It's possible a chipped spider gear could cause the problem... if it gets to one particular gear alignment and pops out, that could do it, I suppose, but there's no way to know without removing the cover.