At 150K it's a good time to overhaul the entire valve body, IMO. The various valves and springs do develop wear and can produce erratic shifts, which (again IMO) subjects the powertrain to unnecessary stress. As Shogun said, the major gaskets also get brittle from heat and will need to be replaced.
With all the great step-by-step writeups available , it's not that tough a job, just tedious and kind of messy. And yes, the check balls are buried deep in the valve body. Once you set out to do the job, there's no point in doing it halfway.
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