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.
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