Diagnosing Driveshaft Vibration

There is a driveline vibration that I can feel mostly when slowing down but sometimes at other points.  If I push in the clutch when the vibration is occurring it does not stop.  This gives me the impression it is not related to the engine or transmission.  My suspicion is that it is the driveshaft yoke or length.

It is unfortunately almost impossible to get pictures of the driveshaft and yoke installed as the vehicle is so low to the ground.  However, I was able to get the following picture of the yoke in the transmission while at ride height.  The worn section where there is no paint is .375" wide and represents the amount of movement occurring.  Many say you need .75" free play at the transmission and I guess that is a safe number as it is double the movement I see.

The trouble is that after taking this picture, I disconnected the driveshaft from the rear axle.  I then pushed it forward to see how far it would go into the transmission.  The driveshaft slide all the way forward until the neck of the yoke touched the transmission seal.  That is a total of 1.7" of movement which is way more than the .75" that is recommended.  

I took the yoke out of the transmission and measured it.  I put all of the measurements below so I can reference them and determine what I want to do.  The best replacement part appears to be a Spicer 2-3-4871X which is identical except it eliminates the end of the yoke which has no splines.  I'd planned to start by just replacing it until I noticed the driveshaft is way too short.  

I then started looking into a new driveshaft and found a few different options:
1) I called a local shop to price extending my current driveshaft from 50" center-to-center up to 51" center-to-center.  They said it would be $280 using my existing yoke and u-joints.  This is rather expensive.

2) I've considered a Denny's Driveshafts replacement as it is a 3" aluminum driveshaft, new u-joints, and the new Spicer yoke.  The tough part is that it is $450 but that is only about $100 more than the local price after I add a new yoke into the deal.

3) Some have purchased a stock driveshaft for a 2006+ explorer (4door, V6, AT, 2 wheel drive) and it is an aluminum version that is already the proper length.  From what I understand, they are sold rather inexpensively.  I'd have to add a yoke and new u-joints to the price as they use 1330 u-joints.  I'd also have to play the local place to balance it which based on the other prices would still be at least $100.  

https://www.vintage-mustang.com/threads/explorer-aluminum-driveshaft-fixed-65-vibration.910770/

I'm going to think on it for now as it is not horrible but is something I want to correct soon.

12/06/19 - After thinking on this a bit, I just decided to bit the bullet and get a Denny's 3.5" aluminum driveshaft.  I then don't have to worry or think about it.  It will just be done right.  It will take about 4 weeks for them to build it so it should be ready around the beginning of next month.