While it is a "lifetime fluid", your looking at 16+ years on this fluid with your '98. For $40 or so you can add new fluid that will last for as long as you own the truck.
Also, have you even checked the fluid level in the diff? Could be low from a small leak (or underfilled from the factory?).
Could you not change the fluid and be fine, more than likely yes, but again, it won't hurt and will give you peace of mind.
Now if you tow a boat or off road and get the rear end in deep water (mid axle or higher) or tow/haul heavy loads a lot you probably should change it every few years (or more often if you do it a lot) since water and gear oil (even synthetic) don't get along.
2002 F150 King Ranch Supercrew 5.4