Don't know the Manual trans nearly as well as the Auto. It uses a master/slave clutch set up correct? Look under the hood for a smaller brake master cylinder looking thing on the firewall about where the clutch pedal is mounted on the other side. This is the Clutch Master Cylinder. Check the fluid level and quality (full/clear). Only fill with DOT 3 rated brake fluid. If the fluid is low there is a reason for that. Move on down.
SET PARKING BRAKE, BLOCK WHEELS ON BOTH SIDES, AND TRUCK OFF!
You will not believe how many people roll over themselves because they're lazy
Have a friend read you each step, good to have one anyway for later on in the list and to hand you tools and beer
From under the truck position yourself so that you are looking at the drivers side of the transmission case/housing. Put your hand on it. Close to where it starts to bellow out going toward the motor will be a roughly 3"x3" square rubber dust boot sticking out of the case parallel to the ground, perpendicular to the transmission. Put your hand on it. Sticking out of the boot is the clutch fork. Put your hand on it. Attached to the fork near the end and running forward to another mount is the Clutch Slave Cylinder. Put your hand on it. Between the Clutch Fork and the the Clutch Slave Cylinder is an Actuator Rod mostly housed in a rubber dust boot of it's own. Is the boot still intact? Pull boot toward the Clutch Fork (off the Slave Cylinder) to reveal more of the Rod and the inside of the boot (wear safety glasses and keep your mouth closed). Is it clean and free of oil inside the boot (using a light look inside of the Slave Cylinder again for leaking fluid and dirt)? If yes, move on. If no, remove Slave Cylinder from vehicle, go to Ford and order a Clutch Slave Cylinder Kit [P/N 7560]. Rebuild Slave Cylinder, re-install Slave Cylinder, bleed Slave Cylinder like you would the brakes. Drive safely.
If you said yes right above ^^^ stay under the truck and have a friend sit in the driver seat pressing and releasing the clutch peddle. Check movement of the Clutch Fork. Is there good movement of more then 1.5-2" at the end of the fork? If yes, most likely bad internal transmission ball detente and keeping you from selecting a gear. The transmission thinks its in a gear and won't let you into the next. It's a safety detente (like a check-ball) ball that rides on a spring and keeps the manual gears from falling into gear while you are already in one (so that while in second, third won't accidentally engage). If no, most likely bad Master Clutch Cylinder up top. Replace, fill, and bleed like a brake system.
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