GM-GMC: transmission 1987 6.2ltr 4wd GMC sierra clasic suburban, fuel injection pump, gmc sierra

Question
the transmission does not shift into high gear until I have been traveling 2-3 miles. transmission fluid is clean. I changed fluid and filter.fluid is not leaking. can travel at 35 mph when transmission is cold on level or descending road (no load) accelerater 1/2 depressed, but when going up incline or depressing accelerator transmission goes into lower gear. Transmission functions normal when warm.

Answer
You did not specify the transmission, so I will assume you have a TH400 as does my my 1984 K-2500.

Check the vacuum system parts.  You must have a functioning vacuum pump, transmission modulator, good hose connections, and a properly adjusted vacuum switch that mounts on the passenger side of the fuel injection pump.

The kickdown switch is electric and mounted on the accelerator pedal.  It is normal for the transmission to downshift when the pedal is depressed to a certain point.

These issues are covered in detail by the shop manual for your vehicle.