Went to shift out of 4th - Stick won't move (pushed pretty hard). Happened all-of-a-sudden, no warning. Stopped engine, started engine, clutch in, clutch out - nuttin made a difference. Ended up in 4L so I could continue up hill to get home.
I'm hoping for a quick, cheap, easy fix that I can perform myself at home - but aren't we all.

 

Maybe the gear shifter bushings are worn out. Take the boot off and check for extra play in the shifter movement. If te bushings wear out, the shifter can get in a bind. Try v6f150.net. I remember I saw a how-to article over there that showed how to disassemble and replace the shifter bushings and whatnot. I think it was under a thread called "tranny fluid change" by a dude named sawblade. He took his shifter off to make changing tranny fluid a little easier, and he replaced the bushings in the process.