Only 2-3 shift is harsh? My 1500 Silverado does this and it's worse on the 1-2 shift, less so on the 2-3 shift. I've got a P1870 code. Mine only does it when I get on the highway for about 20 minutes and get back on the street...
I believe it's the valve body. You can get a replacement valve body for around from Rockauto for $200 and try to replace it yourself. There's videos out there, but the tranny is always on a bench. There's also a kit where you ream out a hole...I don't know much about transmissions, so I don't know what this entails. I think I read on some GM manual or TSB it said if tranny oil pan doesn't have particles you can just do the valve body job...
I called a shop and they said about $600 to fix it...
I hope this helps.