Do you live in the UK mate? I work on Ford Rangers all day long and have done for 2 years now and I've never once seen any transmission problems (apart from a seized front propshaft - but that was on 4x4 obviously). I suppose you could check for any leaks from the gearbox underneath, especially around the bungs as these may of not been replaced properly whilst changing the transmission fluid. Of the top of my head I think the gears are a bit sticky but thats just the feel of the car. The engines are bombproof (Mazda), only thing to watch for on rangers is front wheel bearings, seized brake calipers and rear suspension components in my experience.