1995 Blazer 4x4

O.K. I've got a 1995 Blazer with the 3 button switch that I'm working on and it will not shift into 4-high or 4-low unless it's in neutral. This set-up should shift on the fly into 4-high right? If so, what is my first thing to check?

Make sure all 4 tires are the same manufacturer, same size, and same air pressure.
Make sure you have manifold vacuum at the actuator under the battery, (engine at idle) when you select 4WD.
Post your results.

Make sure all 4 tires are the same manufacturer, same size, and same air pressure.
Make sure you have manifold vacuum at the actuator under the battery, (engine at idle) when you select 4WD.
Post your results.

Engine at idle? Shouldn't this vehicle shift in and out of 4H going down the road?

Yes, but it's kinda hard to check vacuum at the actuator when you're going down the road ;) The actuator must have vacuum when you select 4WD to pull the cable and engage the clutch on the axle shaft. When the clutch is fully engaged, the switch on the right front axle tube is satisfied and it stays in 4WD. If the switch is not satisfied, the TCCM will command the encoder motor to shift back to 2WD.

You might want to check for oil in the vacuum line to the actuator. If the vacuum switch on the transfer case leaks, it will suck fluid into the vacuum system for the 4WD and HVAC system. If/when this happens, the actuator will not get full vacuum and operate slowly, or not at all. You can pull the vacuum line at the reservoir and apply vacuum with a hand vacuum pump and check for leaks.