If you're leaking fluid from either inner strut seals or hubs seal,then I'd say yes replace the seals,armature plates,bearings if needed,but if the awd is ok,just change the fluid every fifty hours or so. As far as u joints I've seen original ones on machines that had several thousand miles on them and still tight. Other machines I've replaced a lot quicker when people never put a grease gun to the fittings. Just preventative maintenance. Plus one machine may lead an easy life,one may be a work machine or constantly in mud or under heavy loads. You'll get different answers from everyone on how long they will last. Always replace both u joints on a common shaft (prop shaft or the older rear u joint drive shafts. It just puts more pressure on the other one,then you'll be replacing it shortly. There's no written in stone "average" mileage you should get out of u joint or cv joints for that matter.

 

Your fan can spin what seems like "plenty fast"...mine seem fast enough also...but your machine is as old as mine is maybe 1 yr difference, and when I bench tested the new fan to the old one it was a HUGE difference.
So on to the head gasket thing...what makes you think you have a bad gasket ? any air bubbling into the radiator (with the cap off you would see air bubbles), no water in the oil....just not sure what branch you are on to inspire the thought of a bad head gasket. Go with the easiest fix first...and that is a simple 20 dollar fan...it is after all over 10 yrs old...how much wear, snow, mud, rain and washings has it been exposed to? not to mention normal cycling of it on and off.
Its a 20 dollar part and an hour of labor to change it...all I'm saying is its an easy part to install and cheap to do.