lost 3rd gear totally. High mileage, hard, hard miles! Do i repair or buy lower mileage replacement? 5.3Liter,Auto. 4x4
If you buy a low mileage replacement transmission be sure it has a warranty. Have you talked to an independent transmission shop to have your transmission rebuilt?
The only real way to know for sure is to tear down your transmission to know what all is wrong with it and what other than the basic normal stuff if any require replacing also… remember that a used trans is used… you could get one that last another 50K or you could get one that is not much more than a rebuildable core…
But I would at least pull the pan and filter to see what it looks like and replace the filter and replace the front pump seal and the rear main (engine) seal, and any other seals that maybe leaking… And use the correct ATF, not some universal stuff…