OK, I have 146,000 miles on my van, it ran like a scalded dog, I was having issues with delay going into gear when you shifted into drive, diagnosis was the transmission needed to be rebuilt. During removal, they found a crack in my case, so they replaced it, I never had a leak, but never the less, rebuilding that one wasn't a good idea, once I got it home, I noticed the whine was gone, evidently the previous one was responsible for a whine I've heard since I bought the van, and the check engine light was on, along with a miss. Now I'm bothered about it because this wasn't a problem until after the replacement, now it's driving me nuts, the code is both banks runnin lean, PCV was good, the rubber elbow is soft, could be collapsing under vacuum, but not leaking. The o2 sensors are good, one is brand new, I replaced it for good measure, no change after resetting the codes, they came back. I'm currently lookin for another rubber connection for the PCV, the ones at the parts stores are too short, the elbow required is tall, to miss the injector wire loom thats secured by the cover, no way to fudge it either. I had the MAF pulled, cleaned it and the temp probe and immediately it was missing on the way home, it starts at about 55 MPH especially when overdrive kicks in, yet is there when I turn it off too now.

 

Well, I took care of the PCV, doubt that was it, the hose was collapsing, not leaking, I bought a hose for a GM, since the Ford replacements were too short, I couldn't tell if it was missing or not, but the check engine light immediately came on, so no fix.
On another note, my wheelchair lift broke, so it looks like driving the van is out for a while.

 

Have you checked the fuel pressure yet? It might be wise to have a shop with a scan tool to drive it and log the data to see what is actually going on. It might cost a bit, but is better than throwing parts at it.
What year is the van? Sounds like you have covered the common issues with a dirty MAF or a PCV elbow. I would go ahead and replace the pcv valve itself if you haven't already. They are cheap and should be replaced every 100k miles anyway.
To check for vacuum leaks, I like to use a small, UNLIT, propane torch and go along the intake where it meets the heads, around the throttlebody and the air intake. If the idle changes anywhere, then you have found a leak.
I would first, check the fuel pressure first. It should be at least 32psi at idle and can go up to 40 to 45psi with no vacuum to the regulator.

 

I have a fuel filter, was going to take care of it, since the rest have been covered, I did the PCV, that's how I found the elbow was collapsed, my wheelchair lift broke, leaving me extremely pissed off, and one more thing to fix. Off to driving my 73 Econoline, till I can chase wiring and switches on my 17 year old lift, so I can get it working, where I can drive the 99 E-350 and fix the miss.