98 Blazer 4x4 disengaging problem

Hi. I have a chevy 98 blazer with an electronic switch for shifting into 4x4.
Every since a couple of months ago when trying to disengage from 4x4, the right front wheel seems to stay engaged for up to 10 seconds, but then I hear a loud thump in the right front side of my truck and the wheel disengages.
This only happens on the right side the left side seems to be working fine.
Any help is greatly appreciated.

The right front axle has a clutch in the axle tube that is operated by a vacuum actuator located under the battery. Slow disengagement can be caused by one or more of several things. The vacuum vent line may be partially restricted, the shift fork mechanism might be binding, the return spring may be broken, the actuator cable may be binding, or the clutch gear or clutch shaft may be damaged. Here is a link that shows the front axle: http://img413.imageshack.us/my.php?image=frontdifferentialxw4.jpg You can check the vent line for restriction by disconnecting the vacuum line from the actuator and connecting the line to a vacuum gauge. With the engine idling, transmission in PARK, shift into 4WD. The vacuum line should show manifold vacuum immediately. Shift back to 2WD, the vacuum should drop to zero immediately. A fairly common problem is the transmission rear seal leaks fluid into the transfer case. If the fluid level gets too high in the transfer case, and the seals in the vacuum switch don't seal tightly, fluid can enter the vacuum lines and cause slow or no front axle engagement. Post your results.

Thanks old_master.
The 4x4 engages fine on both sides. When taking it back to 2x4 I hear a humming sound which gets louder and then big thump and the right tire gets disengaged. Could this be a vacuum switch? How do I test if the TC Vacuum switch is good or bad?
Thanks again for your help

The left axle is engaged with the differential all the time. Do the test I described in post #2. That will test the vacuum lines and the switch.