In my case, I've found the most miserable repairs come after about 150K miles. Guess that depends on driving conditions, climate, salt, quality of former repair, etc. But at that high mileage you are most likely to be confronted with an expensive repair. Then you have a choice to make: Throw money at it that won't be repaid or junk it.
The choice depends on your financial position. You can keep this thing alive for another 60K miles, at least, but it will cost you in reliability and work. You will start spending money on parts and labor. If you have the money to burn, you can avoid all that with a 30,000 mile truck (for $25,000) that will last till you're 80 with few problems or repairs.
There are your choices. I'm at that point too, Brother. It's "payments or parts" as they say.
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Preventative work at 140K miles? Change tranny and transfer case fluid. See how bad the rust is on the rocker panels, radiator support and body mounts. Check the brake and trans lines for rust. Check brake hoses. Lower the spare tire. Look for head gasket leaks. Listen for timing chain rattle. Are the plastic harness plugs falling apart? If you look well enough, you may find that truck has earned its retirement also.