I bought a 2009 PK Ford Ranger XLT 4WD auto to replace a Ford AU11 ute to tow a horse float weighing 900 kg and two horses weighing a maximum of 1000 kg. Up until the Ranger's 30,000 km service I was very happy with the vehicle, it had power up the hills to my property and I could comfortably do 95 k/h up the steepest hills towing just under two tonnes. I did a trip three days before the service without a problem, but three days after the service I did the same trip with the same load and couldn't reach 70 km/h up the hills! In fact the Ranger was totally gutless and wouldn't even kickback a gear. The Ford dealer couldn't find anything wrong and couldn't understand why there was allegedly such a difference in performance after the service. The problem has been addressed a couple of times now by the dealer with only marginal improvement. With the latest adjustments I noticed a big increase in general torque around town, not towing anything, but on any road with a hill it's embarrassing. I've been using my husband's 2011 Toyota Hilux SR5 4WD for towing for the past two months and it is far superior to the Ranger with plenty of power when you need it. Go figure?
It would seem something has gone wrong, perhaps at the service, as you seem to suggest. It's most likely a fuel supply problem, so I would suggest you have a diesel engine specialist check it.