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When I drive along a smooth straight road at about 35 - 40 mph, I sometimes notice that my truck seems to lunge / surge fore and aft. Does not seem like fuel / engine problem, but may be related to a shimmy / shudder that I posted a couple of posts back. Truck has a decent vibration upon acceleration from a stop up to about 15 mph, gets worse when towing 8000 # load. Truck has 51K miles. Have also noticed a little bounce in the speedometer at highway speeds, probably about 3 mph.
Have you changed the fluid yet? Have you checked for codes? Have you tested the MLPS? If those don't pan out I would start to lean toward the torque converter. But definitely look at everything else 1st.