My method is to gauge if the car still does its job. To me, that job is to get me to work & back without breaking down.
Last couple cars have been Toyotas or Hondas, so they last a long time, based on reputation. So I am willing to keep a more reliable car longer, because it will likely do its job for a longer time.
I also base my decision on how many and what kind of repairs the car has had while I owned it, and anything I can learn about the time before I owned it. If I feel a car is unreliable based on the types of repairs, then I will get rid of it sooner. Alternatively, if I know that once X repair is done, it will now be good for Y years/miles, then I want to get the most out of that repair & that will factor into how much longer I feel I can keep it.
It's tough, because there is an emotional component, too. My cousin had an SUV she really loved, but it was having major mechanical issues. They were eventually fixed for beucoup bucks, but I urged her to sell due to the nature of the problems. Sure enough, the thing is a brick in her driveway now. But she loved it so much it clouded her judgment.
It kind of sounds like your Ranger isn't doing the job of towing too well anymore...how much do you tow these heavy loads? Does it do what you need otherwise? 176K miles is kind of a lot, but I'm not really into trucks so don't know if that's really the end if its lifespan.