4 wheel high pops out Trooper

Hi, I've been looking for 1986 Trooper 2 door with low mileage. At the end of the long search I finally found one with only 95k. It had been off the road for 7 years. Carfax confirms. When I went to test drive it
everything was great until I attempted to use the 4 wheel drive. It is a deluxe with auto locking hubs. I put it in 4H. The light came on on the dash. I would drive about 20 feet and it would pop out back into 2wd with a loud clunk. Is this a major problem that will require lots of work or something simple. Oh it had no trouble in 4L. Thanks, Joe

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