My drive shaft had a weep hole in the end to bleed excess grease out. Doesn't work if you have petrified grease though.

 

I am no where near an expert on F series, especially the big jobs, but sometimes having more than 1 set of eyes on a problem helps.

 

Mystery solved. Well enough that I am comfortable. With the truck in the air and the axle hanging the driveshaft is in compression. With the weight on the tires and the springs compressed the rear slip yoke is about 1/4" to 3/8" away from bottoming out the slip joint. I must admit that I did not expect the rear to move that much on the springs. I'd prefer another 1/4" or so of clearance but unless I get the truck airborne I think I am probably OK.

 

I don't see how that's possible. With the axle hanging, it has to be 3-4" down from where it is when loaded. How could the DS be compressed then, but free when it's up? When the springs are compressed, the axle moves backwards some, but stock springs aren't exactly flexible. What springs do you have? Is this truck lowered?

 

Ross, I agree that it didn't make sense to me either but that's what happens. The de arching of the spring at ride height has a larger effect than I imagined. I couldn't get the rear UJ in the axle yoke with the truck in the air. Putting the truck on the ground gave me more than 1/4" clearance. The springs are stock F3 springs with 11 leaves.