If all characters are over 6' (and too big to fit through the small opening) then one option (if you have the Divine Sigil from Urthdala) is to jump off, slowly move forward, and as soon as you drop down just a little quickly walk backwards. If done correctly you'll back up into the tunnel area past the two blocking boulders and can open the chest and then use the Sigil on the boulders to exit through the tunnels.
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Rick
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