Is this rust something to worry about?

That's what I thought but the dealership said it's all original.

 

The metal and finish look stock, but that weld...

 

I'll post a photo of the same weld after I clean it up.

 

for all things rust:

https://www.tacomaworld.com/threads/rust-prevention-treatment-consolidation-thread.355928/

 

If you park it on a slope when you do it, it works even better.

 

Nothing will turn rust back to metal. Rust can be only removed and/or coated with something that will stop or slow the progression. I don't like hard-shell paint on crappy metal (Tacoma frame is made of crappy metal) as the hard shell paint will cover deterioration progress. So far I'm sold on Fluid Film (that stuff is designed for protection from salt water) which soaks into rusty surface slowing down surface oxidation. Of course if the metal is changing the structure inside, nothing will stop it, but my hope is that Tacoma frames used a bit better metal than for example Fiat in 70s (Fiat's metal was full of sulfur and was rusting under the good paint).

While Fluid film is great inside the frame tubes (flows easy and soaks the metal well) on the outside it does not last more than a year. Car wash or driving through the water will eventually remove the layer of fluid fill. So it requires to be re-applied at least once a year. It should become a routine maintenance item (like oil change) that would prolong the life of the truck.

From all the horror stories I see that untreated (not maintained) Tacoma frame will last 6-8 years before showing perforation and failing inspection. So far my Tacoma is 2 years in "salt on the road" environment (after the move from California) and I see no sign of new rust. I'm doing fluid film every year so I guess I will report back in 6 years how efficient the fluid film treatment is.