Dodge D150 1990 Service Manual page 156

Engine vibration f a u l t s can be located by accelerat-
i n g the engine
(transmission
i n " N E U T R A L " )
through the speed range and noting the speed when
(if) the vibration occurs.
Drive shaft vibration will increase i n intensity as
the vehicle speed is increased and it is not sensitive
to engine torque. A vibration that occurs within a
specific speed range ((e.g., 55 to 65 mph or 88 to 105
km/h) and does not occur above that speed range is
not caused by drive shaft unbalance. Defective U -
joints or an incorrect drive shaft angle are usually
the cause of vibration that occurs within a specific
speed range.