u had to change mine out after i broke my rear yoke...and had to get a drive shaft...but that was at 110XXX miles...my step dads f150 has 210XXX miles and still has the factory u joints and they are fine...even factory clutch
Driving style/heavy loads ect. affect the life. My dads '94 got changed @ about 160K though the truck often towed more than it should have.
My '95 still has the original u joints, and clutch for that matter and has 200K
I have been thinking of replacing them but....