2005 Ford Freestar stuck in park.

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My wife has a 2005 Ford Freestar. The gear selector lever moves just fine and the linkage is moving at the neutral safety switch but the transmission stays in park. You can here a clicking from the neutral safety switch location as the gear selector is moved in to each position but the van stays in park. Does anyone have an idea whether the neutral switch could be the problem or if it is more likely to be the transmission itself? I can't seem to get replies to post here. The temperature here has gone up about 20-30 degrees F and the van is now working just fine. I have no explanation for that but I'm going to try replacing the transmission filter and fluid (w/o doing a flush as recommended in a reply I got that is not showing up here) and see if that prevents the problem in the future. If the site starts working better I'll update. Thank you pushrod for your suggestions, they are greatly appreciated!

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