SOLVED: indeed turned out to be the YAW Sensor / Speed Sensor / Acceleration Sensor (BMW # 34526777435)
Last weekend I replaced the sensor which is under the center console. Did not require any programming or coding. Just swap the sensor and you are back in business.
I was able to remove the screws from sides of center console which then allowed it to lift just enough to rest it on a plastic bowl I had (5-6" tall). Then I could use a 1/4" socket (10mm) and unbolt the sensor and wiggle it out.
To recap.
*Only warning light I ever had was 4x4 light in center of dash (knows as DSC warning light).
*ABS lights were OFF
*4WD still appeared to work but I'm not sure
*Warning light was intermittent over a 2 month period (I swear it came on after door slamming
Then light came on solid and stayed.
*GT1/DIS showed DSC internal sensor error
*GT1/DIS showed ZEROS for Rotation rate and Transversal acceleration
*Replaced Sensor with BOSCH branded unit from ECS Tuning. Pictures look to be exact match from what others have bought from BMW.
Thank you all for the info and replies. Thank you TwinsPoppa for your original post that put me on the right track here. Only difference was I never had self level suspension errors but otherwise the same. Also used your console remove directions.
I learned a bunch of GT1 use here and would be happy to hear questions from the next 'guy'.