Not “all” FDs were built with too much preload. Clearly, far too many were, but there are many stories of factory FDs that ran a long time without failure.You never say how many miles are on your final drive. I assume a 1999 model may have quite a few miles. It will fail eventually without a rebuild. I was lucky and got 65,000 miles from mine before it failed. All of a sudden you hear a noise in the final drive you pull over and oil starts coming out. All of the drives in the LT were built with too much pre-load. This causes the metal to fatigue and then the crown wheel bearing begins to come apart. The pieces get ground up and puncture the seal and get trapped by the magnet.
If you are going to keep your LT, plan on rebuilding the final drive sometime in the future. As others have said change the final drive oil every time you change oil and pay attention to the magnet on the drain plug.