It's possible that your clutch friction material has been contaminated and glazed up.
My truck has Fluid Drive with a 4 speed. A couple years after I finished my rebuild I noticed the clutch slipping in 4 gear, on a hard pull. When I was putting it together I cleaned up the clutch disc with solvent, which effected the clutch material over time. I pulled the clutch and had the pressure plate rebuilt, FD clutch plate resurfaced, and reassembled with a new disc. It's been fine ever since.
I would advise caution when working with the FD unit. I was naive at the time, when I pulled the clutch plate from my FD. I learned later that this type of work could potentially damage the seal. But my work method turned out to be decent and no damage was done. I also drained and refilled the FD when I was reassembling the engine/trans. I used AW32 Tractor Hydraulic fluid, which seems to be working well. Also, since that time I've seen several posts here regarding leaving it alone unless you are having an issue.