I picked up a jack shaft from Wreckerman last week (Thanks Buddy!) and got a chance to install it this week. Mine was shot but I didn't realize how shot until I got to see one that was in good condition. It wasn't until I got the old one out that I realized that it was not long before it would've failed completely. When I got the truck, the shaft was bone dry and rusty and the spline seal was cracked and no longer touching the inner shaft. I had run about 2 tubes of grease through it to clean it out but the damage had already been done.
The saving grace here is that both U-joints were new with crystal blue grease in them. So I replaced the shaft and reused my U-joints. Apparently the U-joints had been replaced but the shaft was left alone. While under there, I checked the rest of the drive line and found that ALL the U-joints had been replaced and the rest of the shafts in the driveline where fine.
Pictures of the new one...
A couple of notes for others...
The old shaft was a pain to get out. The old grease in the bottom of the hollow shaft didn't want to let me compress the shaft all the way. Much prying, banging and the excessive use of choice words were necessary. It will come out though if compressed all the way. Trans and transfer case in neutral will allow you to position it however you need.
The replacement shaft had the same problem with the old grease. I pulled it apart and cleaned it spotless. Be careful with the spline seal here. The splines must spread the seal in order to remove the inner shaft. Some grease on the splines and seal and a blunt pick helped manipulate the seal during removal and assembly. Do not refill with grease until after you get it installed. You'll lose it when you compress it for installation and have to add more anyways. It is a good time to grease the U-joints though since they are easily accessible.
The old shaft was installed with the spline seal facing the transmission or front of the truck. I installed the replacement with it facing the transfer case or the rear of the truck. I can only see this helping keep dirt, dust and road spray out of the seal with it facing the rear. I honestly don't know which way it was from the factory since it had been removed before. I don't see any reason it can't be run either way though. This little shaft needs all the help it can get. It's the only shaft that sees the brunt of all the torque whether in 6x6 or not. There is no torsional relief to dissipated any shock loading because it's so short.
After replacement, all the clunking during direction changes and downshifts is gone. All the drive line vibrations are gone now too.