Hi,
Yes, you do have to remove at least one seat, the center dash brace, carpet and the gearbox cover. (I've seen modified, two-piece covers that make it a little easier, but would assume you have a one-piece cover.) It is impossible to get to the nuts and bolts from below. In fact, they are hard to get at in any event, you'll probably need to use open-end wrenches on them.
In fact, during regular service you need to be able to access the front u-joint from above - via a large rubber plug near the rear of the gearbox cover - to grease the u-joint. This doesn't require a lot of disassembly, but also doesn't give enough access to allow removal of the nuts and bolts.
If your drive shaft is clunking, it could be the u-joints or it's also possible the old nuts and bolts have gotten really loose. I found 6 of 8 were less than finger tight on my car not too long ago. Pretty scary, the thought of a drive shaft coming loose on the road and flailing around.
The original nuts were Nylock, and really shouldn't be reused in this application. At the very least, buy eight new 3/8" NF Nylock nuts.
I highly recommend going a little further and getting an upgrade bolt and nut kit that The Roadster Factory offers. It includes all-metal self locking nuts and grade 8 bolts. I found the bolts provided a little long for comfort at the diff and cut them down a few threads to increase clearance slightly. I also used Locktite on them just to be sure.
If you find the old bolts loose, the u-joints should probably be replaced as a precaution. The front splined section can be removed for cleaning, but must be re-installed in the correct orientation. There are arrows stamped into both front and back sections of the drive shaft that should match up when done. Inside the cup that screws on and holds the front drive shaft section to the rear is a cork grease seal that often falls apart during disassmebly. It's not available from any of the major TR vendors, but there was discussion here about it in the past and someone mentioned a source. I ended up making a seal out of sheet rubber that seems to work.
It's not a bad idea, too, to have the rebuilt drive shaft professionally balanced. You can probably find a local shop to do this.
There has been some discussion of good and bad quality u-joints in the past, so you might look in the archives of this forum. I fitted greasable ones, G&K if I remember correctly. Not sure where I got them, though. They had been on my shelf for 4 or 5 years.
They seemed well made and fitted nicely. The factory manual was helpful fitting them. Someone mentioned not using a hammer, probably good advice but unfortunately I didn't really have a choice when removing the 25 year old, non-greasable u-joints from my car! I was very careful to avoid damaging the yokes in any way.
Hope this helps!
Alan
Hey there Guest!
Hey - did you know if you click on the title of a thread it will take you to the first unread post since you last visited that thread?
but were afraid to ask: 
STOP!! Never post your email address in open forums. Bots can "harvest" your email! If you must share your email use a Private Message or use the 
smilie in place of the real @
Pretty Please - add it to our Events forum(s) and add to the calendar! >> 
A friendly reminder - be careful what links you click on here. If a link is posted by someone you don't know, or the URL looks fishy, DON'T CLICK. Spammers sometimes post links that lead to sites that can infect your computer, so be mindful what you click.