98 Blazer 4X4 problem

My 98 Blazer won't go into 4 wheel drive. When I push the button, the light flashes, it clicks, and nothing happens. I cleaned the plug on the control box. I replaced the vacuum switch on the transfer case. I checked the vacuum thing under the battery box, but there's no vacuum there. The motor on the transfer case looks like someone replaced it before me. What else could it be?
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3 button or 4 button Auto 4WD? Can you hear the transfer case make any noises when you select 4WD? The vacuum switch on the transfer case has 3 vacuum lines attached to it. One goes to the actuator under the battery, one goes to a vent that is attached near the top of the transmission dipstick tube, and the supply line must have vacuum to it when the engine is at idle. Make sure all 3 lines are clear, (no restrictions, & no fluid in them). Post your results.