This truck – at 70k miles – still looks very serviceable to me. Ask your mechanic for a recommendation of an inde mechanic who has the tooling to work on differentials. It looks to me like it can be repaired at least well enough that it will work fine, but you might have to check and top off the fluid level once in a while is all. A little leak from a differential isn’t that unusual. Plus it appears the leak is from the threads on the plug not sealing, which is easily rectified by the RTV method posted by @cwatkin above. I’ve fixed a similar problem just wrapping the plug threads w/a layer of teflon tape like plumbers use. If you see signs of leakage through porous metal in the housing, Google that problem. I expect there are simple methods to minimize the flow rate of the leak to the point where it would be acceptable (for a used truck that is).
If the differential seals need replacement, even that isn’t that big of a job for an inde mechanic with the right tooling.
Do you have one of those do-it-yourself car washes in your area where you put some quarters in the meter and they provide a pressurized wand that spray water at a high force? Those work very well at removing salt from the underframe and wheel wells. During the winter when I lived in snowly Colorado I used one of those car washes every week to remove road salt from under my Ford truck, and the truck (early 70’s) – a little the worse for wear – is still and on the road.
On the other hand, if you are looking for an excuse to buy a new truck, I think we can agree you have one. Your truck isn’t in as good condition as would be a new truck. Some teenager in town would probably love to buy it from you. And you’d contribute to the economy and provide jobs by buying a new truck. It’s a compromise.