Getting more and more difficult to get my truck into gear

For the last couple of weeks it has been getting more and more diffucult to get my truck into gear. Prticularly from neutral into first and neutral into reverse. I have to use both hands and put ALL my weight into it practically breaking a sweat. Some think it's a problem with the slave cylinder, but my clutch doesn't feel mushy, it feels great. There is plenty of give and take between the clutch and gas as I move from gear to gear. A mechanic said that if I try shifting from neutral to reverse while the engine is on and then turn it off while I am trying to shift and it goes in smoothly it is the clutch, but that didn't happen, it was still a bear to get it into reverse. It was also thought that there was a leak in the resivoir that is located driver side, ( I don't know what it is but it has a black cap and takes break fluid), but that resivoir was full, it only took about a table spoon of fluid and shifting didn't get any better. (And of corse I dropped the cap into the engine somewhere, awesome:()Though I don't know really anything about engines, my gut tells me that it's just that something isn't lined up right and/or needs lubrication. Or maybe I just need a transmission flush rather than a new slave and a new clutch. I have also been told that a transmission flush is never necessary. Argh! I have no idea. Can anyone help? I don't want to be taken advantage of at a repair shop with unnecessary things to be fixed.