change the gearbox oil it is ATF you have sticking valves due to dirty oil. and oil all linkages.
A 1999 Suburban may stop shifting when it warms up due to several potential issues, such as low transmission fluid levels, a failing transmission solenoid, or overheating components. As the vehicle warms, fluid viscosity changes or internal parts may expand, exacerbating any existing problems. Additionally, a faulty transmission control module or electrical issues could disrupt shifting patterns. It’s advisable to have the transmission diagnosed by a professional to pinpoint the exact cause.
Had same problem with 1998 ford contour gl. Replaced transmission range sensor and problem stopped.
Because you don't have enough the transmission fluid in the system.
Try to replace the oil filter if the transmission is not engaging when it's cold.
I would check the transmission fluid level.
drain the gearbox and refill with ATF the valves in the gearbox stick due to dirty ATF.
Check the transmission fluid level (no you cant check it like oil level you need to have it on and in gear (make sure the wheels are blocked and the emergency break is on)). You may want to also check to see if the rubber transmission mount is sagging.
The transmission may not be building up pressure until the fluid warms up. Check the fluid level to make sure that its full, then check the transmission filter as it could be stopped up.
This is commonly caused by dirty fluid and a dirty filter. Drop the transmission pan and replace the filter and refill with new fluid.
the seals in the transmission are shrunk but will expand when the oil gets warm so it will then shift
when the fluid is cold it will not flow through the transmission as well but when heated up it flows freely
mine was acting up too. took me forever to find it. it was the range sensor.