1. Check to see that the Master Cylinder brake fluid reservoir is full and that there are no leaks around the both cylinders. Look for leaks on the firewall or where the slave cylinder connects to the bellhousing.
 2. Make sure the clutch pedal is 1/2"-3/4" above brake pedal. 
 3. If both steps 1 and 2 are okay and before you remove the transmission, you can manually test the clutch with an "H" bar style steering wheel puller.¹
 4. Remove the Slave Cylinder from the bell housing² and attach the steering wheel puller in its place. Screw in the threaded rod of the puller and tighten it into the clutch release fork where the slave cylinder rod sits. Once the threaded rod is up against the clutch fork, use a wrench to turn the rod in 7/8ths of an inch and stop. Start up the engine and see if the clutch releases and you can shift into gear. 
 5. If it's working fine the hydraulics may need to be bled or possibly a new system needs to be installed.
 6. If the problem doesn't change or gets worse, then the transmission will have to be removed. A fluid change on the transmission could be in order.

¹ If you do this with the truck running, make sure to block the tires, and all safety precautions are taken. DO NOT step on the clutch pedal with the slave cylinder removed. The slave rod can over-extend and come out of the bore. Reinstalling the rod is not recommended.

² With the slave cylinder removed, make sure the clutch fork can move back and forth between the pressure plate and bellhousing by 1/2"-3/4"
   If the fork is pinned up against the bellhousing, or the free travel is more than the measurement provide then the transmission will have to be removed and clutch inspected. Call our tech line at 1-732-667-7937. 

For a video on how to bleed this hydraulic system, go here.