Fluid changes do help this problem somewhat. If it is equall resistance in all gears I start w/ changing the fluid. But if it seems worse in underdrive gears, 1-4 & Rev I'd look at hydraulic's & the trans itself. Aside from the shifter detent pieces, the shift rail & forks & bushings, & the bearing endplay & backlash settings in good condition/standing, only other things to worry about are;The clutch teeth on the speed gears the shift collars on the syncro's & the blocker rings have wear on them. That will add to the resistance in shifting when the teeth get rounded off, that it need a new one.