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Hello and welcome. There are several resons that message can come up. a problem in the rear drive clutch pump check valve, certain PCM(powertrain) codes, and also ABS codes. If there is a problem in the abs that requires AWD to disable, you may or may not also get an ABS ligjht on the dash. The first step is to have all the computers scanned for codes to see if the awd was disabled because of another system. You could check the fuse in the dash fuse block labled 'enhanced evap./ awd'.
Also, there is a connector under the van that can cause several different electrical problems. It is a body passthrough under the van, just behind the dr. seat location. There will be 2 or three connectors that plug into the underbody. Just dissconnect those and look in the body and harness connector for signs of corrosion.