1986 Ranger -- question with 4wd "clunking" sound

My wife's 1986 Ranger has a 2.9 V6, POS Mazda 5 speed, and has push button 4 wheel drive with low and high range. My question deals with the drive mechanism in the transfer case. 4X4 works fine but makes a terrible clunk with a lurch about every 40 feet while engaged. Front u-joints are good. It sounds a lot like the drive chain is loading up in the transfer case and then jumping a tooth or two are reseting itself. It does this even on ice so I'd say it's not due to external circumstances. The truck has 127k on it and the drivetrain has never been touched except for u-joints.
Any idea on how to check the transfer case drive chain for slack? I don't have to pull the damn thing do I? Thanks for any help.
Troy
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