Questions are in bold.
I am considering putting a 2wd ZF-5 in my 1996 E150 4.9 and am looking for driveshaft advice. The E150 has stock suspension with a flanged 8.8 3.08. The length between u-joint centers will be right around 72".

Is the flange or yoke better on the differential? Is it easier to have things off balance with the flange?

I currently have an M5OD in there. The 3.5" steel driveshaft is right around 72" long and I am pretty sure it vibrates at 62+ mph. I don't really think it is a wheel, but it could be. It was custom built and balanced. The M5OD is a little loose on the tail shaft side of things, and the driveshaft is maybe a 1/4" short (seal is 1 inch from yoke instead of recommended 3/4"). The driveline angles are all within 1/2 degree of canceling each other out when sitting with a normal load.

Should I go with a single or double (carrier bearing) type driveshaft? Carrier bearing is doable - there are holes in the cross member for one, but getting the angles right will ne some work. The M5OD has the slip joint at the transmission, whereas the ZF-5 has the slip joint in the driveshaft. How does this affect vibration? Is the slip joint ZF-5 driveshaft less likely to vibrate?

What diameter shaft should I use? I don't think I will ever go over 220hp, and vehicle weight is usually under 6500 lbs. Rear end is 3.08 right now and I do climb steep driveways. I assume that at 72" I will be better with a 3.5". Is this correct?

Ideally, I'd rather use a single piece since it is cheaper and has fewer parts to go wrong.