Here is an uopdate on 98 NP233 transfer cases:
Info on the 3 port vac switch I bought a Motormite #49315 for $15 at Advance and its on the Help part isle (not listed online so don't try and look it up) lists for 1983 -2000 but think 98 was last year in Blazers. Two ways to look at it so I'll explain looking down on the plastic 3 port switch with the locator raised tab at the bottom, the port to the right is the Vac port from the engine, one to left of tab is a vent port, and the other is the vac to actuator port:
Pic of ports O to front act port
vent port O . O vac port
So when the spring loaded switch ball is down (2wd) the vac from the engine goes no where or is off (other 2 ports are open and connected together). When 4Hi is pressed an arm in the transer case pushes the ball up and the vac is applied to the front actuator port and front axle coupler engages. When 2wd pressed again the vac is removed from the front act port and it vents thru the vent port releasing the actuator. This vent port is T to a rear transfer case vent port also and the line usually runs up the trans dipstick with a small vent on it. The 3 vac lines coming in to the switch are in a formed rubber fitting that keys to the tab on the switch top so hard to get wrong. >>>>Remember that if you look into that rubber connector ports are reversed from my above description<<<<.
This was a NP 233 transfer case 3 button and for all note that the front drive shaft does NOT turn at all in 2wd - I tested and watched.