Why would the shaft between the transmission and transfer case need a slip joint? Both the transmission and the Spicer 23 should be mounted pretty solid to the frame. The slip joints should be installed in the driveshafts coming from the transfer case and going out to the drive axles because of the articulation of the springs and suspension.
I believe the NV4500 would be the eaziest overdrive swap. If your truck is truly scraming at 45 mph perhaps you have the 5.14 gears. You could get gears that are as high as 3.90 to replace them, but those are very rare. 4.11s are much more available. The front and rear axles use the same gearsets, which were basically the same from about 47- 72 3/4 ton trucks.
Changing the gears would affect the offroad character of your truck. For wheeling the lower gears are usually desireable. If you were to change the transmission out to a NV4500, you would lose some of that low gearing as the NV4500 has a higher 1st gear ratio than the sm420... 5.61:1 compared to 7.05:1. The Spicer 23 transfer case is also pretty whimpy with the low range being less than 2:1 ratio. For a few dollars more you could get a married transfercase for the NV4500 with a better reduction for creeping along.
