I have a leaking steering rack on my 2004. I have two questions: How common is this problem and, if uncommon, can I get a used rack and expect reasonable reliability from it?

My truck is a late 2004, 4x4, 4.6L V8 Auto, Xtnded cab.

 

I wouldn't chance a used unit, best bet is a rebuilt one.
Common? How many miles on it?

 

Most "factory" racks have 2 sections of aluminum inside the rack that is machined which is where some of the failures occur. The better reman's put bushings in this area and should last a long time as long as it never runs low on fluid.