I have a 2019 Sierra SLT 4x4, and I lived with the crappy ride for almost a year. I noticed the same thing, it would go sideways when going over bumps, lose rear traction easily, etc. I installed new Bilsteins on the rear about three weeks ago - best $200 I ever spent. The ride is much improved on all less than perfect surfaces.
As others have said, loading up the rear with several hundred pounds and/or reducing tire pressure can help (I tried both) but neither helps nearly as much as getting the junk factory rear shocks out and replacing with good ones.