Today I built a front drive shaft for my 99 mountaineer with the 4406 transfercase. The reason for this shaft upgrade was due to hwy speed vibrations. Since the front axle is live all the time it will always be spinning.
What parts I needed was...
91-94 explorer front drive shaft with double cardan joint
Spicer 209 flange (expensive $70 shipped)
Two new ujoints
You replace the flange that bolts the 1354 transfer case with the spicer 209 flange. The double cardan joint requires two ujoints you have to use ones that do not have grease fittings.
The spicer 209 bolts directly to the 4406 transfer case.
This picture shows the double cardan shaft I built on top vers the plain shaft I had in the mountaineer. So you can see the length is very close.