Check that the reservoir is full, and there are no leaks around the master cylinder on the firewall, or at the slave cylinder at the bellhousing.
On units with adjustable Master Cylinder rod (HYDX1.50 and HYDX.750) Make sure clutch pedal is 1/2"-3/4" above brake pedal.
If these two steps check out, before you remove the transmission, you can manually test the clutch with an "H" bar style steering wheel puller. 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.
Now remove the Slave Cylinder from the bell housing** and attach the steering wheel puller in its place. Using the threaded rod of the puller, tighten it by hand into the clutch release fork, where the slave cylinder rod would normally go. Tighten it as much as you can with your hand, then using a wrench turn the rod in 7/8ths of an inch and stop. With the truck running and the puller on, see if it shifts into gear. If it's working fine the hydraulics may need to be bled or possibly a new system needs to be installed. 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.
** With the slave cylinder removed, make sure the clutch fork is able to move between the pressure plate and bellhousing 1/2in-3/4in. 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 1-800-988-4345