If you start to experience any shifting issues on your Ford E40D automatic transmission, or honestly on any automatic transmission. It's always possible that it could simply be a faulty range switch, especially on these transmission where the parts have been know to have a higher than usual failure rate.
Replacing these range switches is quick and easy on the E40D, and usually is on most other transmissions as well. If you're having issues with your transmission seeming to not know what gear it is in or should be, dropping into neutral when you haven't touched the lever, these are all strong indications that your range/neutral safety switch might need replacement. The part is inexpensive and repair can take as little as 10-15 minutes, so give it a shot and call it a job well done.