Okay, I've been looking on here tonight and I see something about a rotoflex coupling. I'm taking my Disco into the drive line shop on Thursday to have the U-Joints replaced and the drive shafts balanced. What is a rotoflex coupling? Do I have one? Should I have the guy order one before I go so he can go ahead and replace that? I'm absolutely terrified my drive shaft is going to fly off my truck and kill me as I drive down the highway. Help.

 

Here is the roto flex. If it is cracked it needs to be replaced.
http://www.roverparts.com/Parts/TVF100010G.cfm
Here is the conversion kit to get rid of the roto flex.
http://www.roverparts.com/Parts/9300.cfm
Just look at the rear diff where the driveshaft connects to the rear diff and look for a u-joint. If you have one then just replace that and you are good to go. If not you need a new roto flex or the kit.

 

If it is cracked are there any detrimental things that can happen besides serious vibration?

 

Just a really bad vibration. Nothing will explode as far as I know. You can replace it yourself once it starts to crack at a later date.

 

Okay, I just crawled up under my truck with a flash light. It looks like I have 4 u-joints. 2 on the front shaft, 2 on the rear. Now, on the rear shaft near the transfer case, there is this rubber gasget looking thing about 3 inches from the transfercase. What is that?

 

Does it look kinda like this?
http://www.roverparts.com/Parts/276483.cfm
If so that is what covers the slip joint on the driveshaft to keep dirt and grime out of it. It should have a grease nipple near it to keep the slip joint lubed.
The slip joint allows the drive shaft to expand and contract (get shorter and longer) with the movement of the axles. Both driveshafts have them.