When you select 4WD HI at the dash switch, the Transfer Case Control Module, (TCCM) checks the Wheel Speed Sensors, (WSS) and the Vehicle Speed Sensor, (VSS) to make sure all wheels are turning at the same speed, (no slippage). Then it commands the encoder motor on the transfer case to shift into 4WD HI. When the linkage inside the transfer case shifts, it operates a vacuum switch that sends manifold vacuum to the actuator under the battery tray. The actuator pulls a cable that operates a clutch mechanism inside the right front axle tube. When the clutch fully engages the axle to the differential, it operates a switch on the axle tube which confirms 4WD to the TCCM, and away you go! If any one of those things fail to report correctly to the TCCM, the TCCM commands the encoder motor to shift back to 2WD HI.
It sounds like your encoder motor is at least trying to shift but, the problem you're experiencing could still be any one of a number of things. A quick, easy check is make sure there's good vacuum at the actuator when shifted to 4WD with the engine at idle. Make sure the cable pulls in about a half inch, maybe three quarters tops. Post your results.