If you can shift smoothly when the engine's turned off, then look at clutch disengagement, or maybe broken disks & stuff like that. Usually the shift lever will move OK just up to the point where the synchro blocks you out.
If the shift lever is uniformly stiff, that sounds like bushings & pivots in the shift linkage. You'll feel excess friction, which feels different than a synchro blocking you out of a gear.
Since the lever itself feels OK when the linkage is disconnected, then it's somewhere up forward. Something dragging where the shifter shaft goes into the transmission?