The clutch in my 1995 GMC Sierra 1500 2WD with 5.0 will not fully disengage. I cannot get the truck in or out of gear. I beleive this is my slave cylinder but i wanted to make sure its not a problem with the clutch master cylinder before i spend the money on a new part. The master cylinder has plenty of fluid in the resevoir. Do I just need to go ahead and replace the slave cylinder/

While the car is parked /switched off are you able to slip in the gears easily ? if so the fault is as you have confirmed of the clutch not able to disengage the transmission drive when the engine starts.
Now to confirm if this is the slave or the master the hydralics must be checked, check to see the movement of the slave, see that the system is bled and there is pressure in the system, make sure there is no leak or spongy feeling., if so it will be good to open and check the system to confrim without deciding to change with a blind feeling. In this case an overhauling of the system will be enough with the repair kit or the slave unit if found to leak.
So before you just go ahead please check to confirm