Master or Slave cylinder?
 

I have a 1999 4x4 6cyl ford ranger and until this happened, no signs of transmission problems. I was driving home today in 5th gear for a while until I hit traffic. When I went to switch to a lower gear I noticed a lot of play in the clutch pedal and it was incredibly difficult to switch into a new gear. Every gear I switched into from that point seemed to not fully engage (high RPMs for little gain in speed). It quickly got so difficult to switch into gear that I pulled over and called for a tow.

When I first pulled over, I did try to move the truck again in 1st and it wouldn't budge.

I checked and the resevoir with brake fluid is still full. Past that I havent checked anything else.

The tow truck driver believed it could be from a lack of fluid possible caused by a failing slave cylinder or master cylinder. My question is how to determine which is the problem, master or slave?

crawl under the truck and look for a leak

My truck has been leaking a little over time. I had air in the master. I looked up "how to bleed ranger master cylinder clutch" I had to remove the unit and burp the line. My main issue was 1st and reverse.

my main issue was first and reverse as well. i had a bad pressure plate in the clutch