If you use it for business, it's "rusted to crud," and it struggles to pull a trailer (for said business?), I'd say it's time to replace it. If those two descriptors are a bit exaggerated, probably wait a year or so, then replace it.
On the "rusted to crud" part: - I was crunched for time so took it to get an oil change about 6 months back. They told me they had to be very careful replacing the oil, as there was "considerable rust" and they were afraid of breaking something on accident. I even had a sway bar link rust through and break (although fixed now).
On the trailer aspect - I only pull a trailer 3 times a year. The rest of the time I can get away with only using the bed. Although I get concerned about getting stranded on the side of a mountain in western NC (out of cell phone coverage) on those occasions that I do pull a trailer.
In my particular situation, on a "cents per mile" basis I'm coming up with it as about a wash:
Option 1 - drive my Ranger till it blows. With babying, I might be able to get another 24k miles out of it. When it blows, sell it for scrap ($100) and buy a $6k truck that I can get 50k miles out of.
Total miles driven - 74k; Total cost - $5,900 = 7.97 cents per mile
Option 2 - sell the Ranger now for maybe $2,000. Use it toward the purchase of a $6k truck that I can get 50k miles out of.
Total miles driven - 50k; Total cost - $4,000 = 8.00 cents per mile
If the Ranger dies before I can put another 24k miles on it, option 2 is better. But sometimes the devil you know is better than the devil you don't, and there's no guarantee I can get 50k miles out of a new truck for $6k. But I have a tendency to analyze things to death.