1991 Nissan Truck clutch? Transmission?

I noticed the other day that my truck wasn't really "grabbing" much in fifth gear, it would just increase the rpm's when I pushed the accelerator down. Today I was heading up the mountain when I noticed it was doing that more frequently and in lower gears. I nursed it home in mostly 2nd and 3rd but when I push the pedal down it doesn't really take off, it acts like it's in neutral. I had the transmission oil changed out and that didn't help (at all). It shifts like everything is fine and isn't making any weird sounds or smells. Is my issue in the transmission or maybe the clutch plate? This truck has been a real trooper and has put up with a lot of abuse (i'm an awful driver) thanks a ton for suggestions.