I have a 98 Ranger, 3.0L, 4WD, manual trans. When I am accelerating in 1st or 2nd gears the truck often gives a slight bump. After 2nd gear it doesn’t seem to do this and I am in 2WD at the time (not 4WD). It is a lot more noticeable in warm weather. My thoughts are that it is the rear differential slipping and I may try using a friction modification additive. Any ideas on other causes or fixes?