to make sure.... (incase I missread anything)
chock your vehicle, jack up both the left or right side tyres.
lock up the park brake and select P on the trans try to turn the front tyre you should get a bit of rotation before the tcase takes up the slack. IF you dont your not in a range, reselect range and try again. then do this in the other range.
that tests proves the functionality of both your center diff and the high/low range engagement, you may need to have someone rock the tyres to get it across ranges.
lock the CDL with the transmission in N and try to turn the front wheel again it should move a little then stop, when this happens release the park brake and you should be able to rotate the wheel again. This proves the integrity of your CDL. you can also check both high and low ranges if you like.
now select low range and start the vehicle DO NOT GET OUT OF THE VEHICLE DURING THIS TEST. Using an outside observer watch the wheels (or if your doing the drivers side eyeball the front and use the mirror to check the rear) select D and then R on the main transmission and the wheels should start rotating. If they dont then your problem is in your transmission. If they do then lower the vehicle after confirming the tcase is definately in low range (or high if you cant get low) by selecting P on the trans and engaging the CDL then rocking the wheels.
Attempt to drive off SLOWLY, if you can hear a ticking grindy noise from the transmission your screwed thats either the pump not priming or failed, the TC lunching itself, a clutch pack or planetry failure, IF you dont hear a thing stop and check the oil in the auto.
Prey for the failed pump/TC as thats relatively cheap to fix.
If youve had any of them fail Id still suggest a total box rebuild as autos dont like metal in the lube.


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