The daily work van. 2008 Ford E350 5.4l transmission = 4R75E
The last few days while cruising along the trany just seems to shift into Neutral all by itself. If I wiggle the shift lever towards a downshift it seems to pick up and run fine. Its been doing it the last two days, only about once per day. Haven't had a chance to get under it yet.
Thoughts? What to check?
Low fluid would be the first thing I would check, loss of pressure will make it fall out of gear.
If it's a very reproducible thing with the shift lever, you might try to check via OBD scan tool what range the trans thinks it is in. There's at least one switch for shifter position feedback on most automatics of that era, and maybe it's just getting worn out.
Looks like I'm starting to have the 4th gear servo failure. 
Is the lever loose and the indicator not lining up with the PRNDL display? 2008 might be digital, so the second part of that may not apply. There is a torx bolt under the dash that loosens with time. If it loosens enough, maybe it could allow the shifter to be in Drive but the cable isn't pulled tight enough to get all the way to the detent. I never had one get that loose, but I had one get loose enough that it annoyed me into fixing it. It made a dramatic difference in feel. That might be a long shot, but it's easy to check.
Fords also have frequent NSS failures. If the sensor (on the side of the transmission where the shifter attaches) fails, it won't send the right info to the ECM to engage the solenoid for that gear.
Basically, I'm saying GearheadE30 has a good next diag step.
I'll drive 20 miles at highway speed and suddenly it will shift to neutral. If i downshift and upshift back, it engages for many more miles.
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