How slow is "slow" in your terms? Is it just a minor bother to you, or is it actually affecting things to the point you feel it must be addressed? That's not something I would be inclined to have the transmission opened up to solve, and I highly doubt it needs a rebuild. Maybe just some minor valve body or solenoid work, but that's nowhere near $1200.
I'd take it to a Nissan dealer, they have equipment that can watch the transmission systems real-time and identify things better than Aamco could I'm sure.
Brent
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