If a 4L60e or other similar electronically controlled transmission, not
to state the obvious but putting a scan tool on it and reading any
codes, and watching live data is the logical thing to do. Many codes do
not turn on the MIL light but will present drivability issues. Otherwise
you are just shooting in the dark.
Hint: the harsh shifts are likely because the transmission is setting
the line pressure to maximum. For a mechanically controlled transmission
it is possible that there is a problem with the pressure regulator.
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