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Out of curiosity how did you check the mounts? did you look at them or have someone switched the gears back and forth as you looked at it?.....or take a jack and place it under the transfer case and jack it up up seeing if you had any play at all on the mount?
74 the only reason i said that was because my 86 back in 05 or so would slam into gear. and would clunk. had someone look at the shaft as i shifted and it would move a little less than 1/8th of a turn before stopping. then a couple month later hit some water spun the tires then once she hit dry pavement it made the rear end exploded so hard the dif cover had a bug dent into it. but a little check like he did eliminated it as a probable problem.
but froto i know if i had life insurance i wouldnt get my wife to step on the brakes lol. but you could improvise with a camera