I hate to be this guy, but I use this van for my band and we're heading out on the road soon so I have to get this fixed.

Basically, there's a play/slop in the driveshaft where the yoke goes into/comes out of the driveshaft. It's not the u joints, I held the yoke and driveshaft and tried to twist to no avail. But the entire driveshaft rotates quite a bit, with a clunk when shifting sometimes, as well as a sporadic strong vibration that isn't consistent with speed, RPM, but goes away when I let off the gas. It went away when I turned overdrive off(not towing, 30mph) and it seemed to stop when I did that.

No tranny slipping, or anything indicating a bad transmission.

Thinking it could be that extension housing bearing?

Any help would be greatly appreciated.

 

When was the last time the differential lube level and condition of the lubricant was checked? If you see those silver filings in the lube you may be a candidate for a differential job.

 

I just bought the van, but the play wasn't towards the rear. The driveshaft/diff was actually very solid so I don't have reason to suspect its the diff.

It only rotates with play when I grab the yoke and shaft and spin it in neutral. The play is coming from the back of the tranny.

 

I'm not sure I understand just where you are measuring "ply".
If you are at the back of the trans, there should be no up and down ply at the output shaft where yoke slips onto it.
Rotational movement is limited by the trans being in park and the differential. If you are getting any kind of rotational ply between the output shaft and the yoke from the driveshaft that slips over the output shaft , one of them is worn.
The extension housing bushing limits the movement of the slip yoke in the trans housing. Usually if that bushing is worn the seal is destroyed and there if fluid loss present.

 

Well there's certainly no fluid loss. I'm thinking the splines on the yoke could be worn out. I out the van in neutral, got under it and twisted the driveshaft. It rotated quite a bit up front where it connects to the tranny.

Like I said, there's a vibration present that isn't speed related. It goes away when I let off the gas. That points to the yoke splines being shot, no?

 

Does the van have a carrier bearing? and I would almost bet that you have a bad u-joint on the front you need to have the trans. in gear giving that it's a manual and check for play

 

Does this vehicle have an automatic trans or manual?
As grindman stated if a manual trans you have to have it in gear to check the fit.
If it's an automatic you have to have it in park. In either case the output shaft is prevented from turning/moving.
The method you are using to check isn't the best.
Again as grindman stated you could have a defective u-joint and they will usually start going to pot with a squeaking noise on take-off and then if not caught and replaced wear to the point of producing a vibration. How did you check them?
Again some E350s did have a driveshaft center bearing and the rubber mounting wears and can also cause a vibration as grindman stated.