Another possibility... There's a coupler on the shifter rod along the steering column, near the firewall, inside the cab. When mine decides not to shift, it is slipping at the coupler. When it completely refuses to grab and shift out of first, I pull over, pop the hood and move both levers back and forth a couple times to free everything up.
I haven't been able to locate a replacement coupler in any of the usual e-catalogs.
