There are the two lines that operate the diaphragm on the axle. Facing the truck
, the right solenoid (pink & black lines) is activated to disconnect the axle and go back into 2WD. The left solenoid (blue & white lines) lock the axle into 4WD. ***Note that the lines are reversed on the diaphragm in this photo, thanks to FORDS incorrect vacuum diagram***
Start by removing the double lines at the solenoids on the cowl.
You'll have to get a vacuum source like a Mighty-Vac hand pump and apply vacuum to the pink and blue lines, one at a time. If one leaks down you'll have to check the line, including the diaphragm, to locate the leak. If vacuum was held, reconnect the bottom half of the line "connector" (black & white lines) to the solenoids and crank the truck. Switching from 2WD to 4WD and back verify each time that there is vacuum on the top nipple of the activated solenoid. If you're in 2WD the right solenoid (pink/black) should be sucking, in 4WD the left (blue/white) should be. If these operate as described then you axle shift fork may be bad. If you're not getting vacuum when you should be, replace the solenoid. That's a quick diagnosis procedure, need more details just ask.
My 2WD solenoid was bad. I got both of the solenoids and the vacuum line harness at the U-Pull-It for $15. I'd hate to think what that would have cost new.
My diaphragm has a very slow leak around the diaphram plunger shaft when in 4WD but it doesn't affect the operation. There's plenty of vacuum to keep it engaged. I don't often use 4WD and the diaphragm is over $100 so I can live with it.