make sure your clutch is in good working order by checking the fluid first. I am not sure if there is an inspection cover that you can take of and look at the pressure plate but if you can see them, diafram fingers should be even and the TO bearing should touch them all equally. Then have a good look at the shifter to make sure nothing is bent or that any of the mounting bushings are damaged. If the clutch and shifter check out ok you need to pull the
trans and have a good
trans guy look at it to make sure there no bent forks or any parts that move the gears are cracked. If that is all in good shape you may have a bad syncro and the
trans will have to be rebuilt, replaced with a rebuilt or buy a low mileage
trans from wrecking yard or take off supplier. If you are in to cool mods you could look into putting in a 6 speed and the go really steep in the rear axle. The V6 5 speed is a durable piece inspite of all the noise it makes but sometimes even the best of things can fail prematurely.