Hi, I’m new here and apologise if I missed a thread on this topic but I have looked and don’t see anything exactly like the experience I am having with my 96 blazer 4am. Essentially what is happening is the transmission will not stay in 4th gear. It slips. It is temperature dependant. I can make a 100 mile trip without any issues so long as it’s below 50 degrees outside. Also it will stay in 4th (OD) (TCC LOCKUP) above a 70 mph or so. I don’t really know where go start with the diagnostic process here. Trans fluid has been serviced regularly and it’s not low it dark looking. I have a high end scan tool that can monitor whatever I need to but I don’t know what I’m looking for. In my opinion the transmission is in good shape. I believe a sensor/solenoid is giving the van innacuratd information somehow but I don’t know exactly which inputs could have an effect like this. I also wonder if this Is possibly a hydraulic issue, maybe in the valve body but it’s all speculation. I have some automotive experience but don’t know much about transmissions and ang help whatsoever would be appreciated.