97 Z-71 rpm limit

We are having problems with our transmission. It is sticking in second. When stepping on the throttle to pass on the interstate, the transmission
will shift to 2nd. This puts the engine at 5,000rpm and it won't accelerate much. It does accelerate but slowly. The dealer is saying that the motor is weak and should pull hard at 5,000rpm in 2nd. I am thinking that the motor is beyond its powerband if revved past 4,000rpm and the acceleration will be weaker. Besides, this motor has 140,000 miles and is beyond its high rpm power days.
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