I'm no expert, but if you read up on the transfer case in your grand, you will find that while it is always in four wheel drive, the trans-case has a front to rear lock up that occurs automatically above 45(est)mph. which engages the frond and rear drive shafts so they move at the same velocity. below 45mph, or under hard acceleration, it dissengages to allow the front, or the rear drive-train to act independantly of one another, allowing better traction at take off or when turning, 4-wheeling, etc.. I am fairly certain that the "clunk" you hear is the trans-case front/rear diff lock-up as you accelerate past 45 mph. as this also happens in my 93 w/ the same drive-train.hope this helps.