If the test still holds true with the wheels on the ground, I would say it is the vacuum disconnect motor or the vac lines to it. This is on the front axle ( item #10 in the picture below )
The line w/ vac on it :
Blue - when in 4WD
Pink - when in 2WD
You can use a mighty-vac or other vac pump to test the lines.
Could be as simple as a cracked vac line.
If not, might want to consider the Posilock system for it vs. purchasing the part ( 100+ from Ford new, or poss problem unit from a bone yard ).
This replaces the vac motor, and is a simple cable to the ear from the front diff. This is just a thought if the vac motor is verified as non operational.
Start off with the vac test on the motor, and see what the next step should be ( repair vac lines or new vac motor / vac solenoids ).
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my truck is MSOF (shifter on the floor)
I got under the truck and had the truck in four wheel drive and the front drive shaft did not turn. I didn't have the truck jacked up, but the engine was running. 4H and 4L are the same. the 4X4 indicator light does come on. Thanx a lot
If you support the front of the truck, turn the key to the run position ( do not start ) and turn the put the truck into 4WD ( 4H or 4L ). If you turn the front drive shaft, does it move ( sounds like it does not ) ? Does it act the same when trying to shift into 4H & 4L ? - Checking if the Shift on the Fly relay has gone south. - Also checking if the Transfer case signal to the GEM is working, this is the 4x4 Low & High indicator switch, which is installed in the transfer case. Is it a case of the front axle does not drive the wheels, or does this include the Cluster does not get the 4x4 ( and Low Range ) indicator illuminated ?
When you say everything down to the front dif seems to be working, do you mean it seems like a case of the hubs are not locking in ? Is it an ESOF ( Electronic Shift on the Fly ) or MSOF ( Mechanical Shift on the Fly ) ?
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