My 97 LS showed exactly the same traits, and what was the problem,you ask? Simple.

The throwout bearing failed, causing the friction plate to come away from the flywheel at an angle (rather than parallel) which result in an accumuation of the remaining friction plate lining between the flywheel and pressure plate.

This meant the clutch, and by extension, the tranny input shaft was not fully disengaging when I depressed the clutch pedal.
The synchromeshes had to "work against" this partial disengagement when trying to match the speeds of both the input and output shafts.
Hence the stiff, virtually impossible shifting.
Order yourself a new ACT or ClutchMasters clutch, one that is appropriate for the amount of power you plan on making, and swap out the dead clutch and throwout bearing. And be sure to resurface the flywheel!

Good Luck.
Dave.