97 Sierra 4L60 transmission shifts hard after driving around 30 or so miles.

[TheNamesJuicy]

Address P1870 first as that is most certainly the problem. I was going to mention it initially but alsaw you didnt mention any codes (and harsh 1-2 shift has lots of causes that have nothing to do with P1870).

Take the truck to a reputable trans shop and have them do the work. They can check all the vehicle's grounds, including the PCM. No need to mess around with it at this point if you're not comfortable doing so.

Thanks for the info. It’s really annoying to deal with but damn it she’s pretty