You could probably do it without raising the car. I find it easier to get the bottom cover off by jacking it up. As a side note, I am pretty thin and I couldn't get to the transfer case without jacking the car up.
For your transmission, I don't consider 100k too long of an interval to drain fluid. That is the normal duration GM recommends for its cars and what BMW recommends for 07 and up. Now at 140k I would side with the dealer. As a side note, I would put this low on my priority list. If it shifts fine wait.
The more I think about it, that clunk sounds more like the driveshaft and guibo. I would ask an indie not the dealer for them to look at it after you fix the expansion tank.