Join Date: Nov 1999
Location: Central California, USA
If the shifter gets dirty, there is a spring plunger inside the shifter that can get stuck and the plunger can fall into the shift mechanism and jam it. You will only be able to get 3rd and 4th
The plunger has a taper to it, surrounded by ball bearings, the bearings are surrounded by a coiled band spring, like a clock spring. the spring pushes in on the balls, the balls push on the plunger, forcing it against the shifter stick.
If dirt gets in there, the the spring won't clamp down the plunger will fall into the shifter and jam it.
It's not the first thing I would check but...
If everything else looks O.K. check the plunger.