Hard shifting after highway driving (1997 Chevrolet Tahoe)?

I have this same problem when I tow a heavy load on warm days.

Same thing...If i turn the engine of, it goes away.

Sounds like there is a problem with the pressure control circuit.

Have your had the codes checked? Not all the transmission codes turn on the
MIL.

I seem to remember some 96-97's had a rough finish in the input shaft. This
would wear the stator support bushings resulting in a TCC that would not lock
up. (code P1870)

Probable cause is the PCS (Pressure Control System) solenoid in the transmission. These tend to get flaky with age and call for a full pressure shift when hot. The part only costs about $30.00 but the trans has to come out to install it and thats expensive. So, I suggest a cheap fix that often works. For about $75-80 dollars buy a good quality trans cooler and install it in-line with the existing trans cooler in the radiator.

While this is being done check the radiator trans cooler very carefully!! If the vehicle is more than a few years old there's a chance that engine coolent is leaking into the trans cooler section. Contamination of the trans fluid with coolent can cause hard shifting and damage to the trans or torque converter.

The cooler is easy enough to install if your handy but it's best to have a good shop do it becasue you'll want to have a full trans service done as well - flush, fluid and filter and remember to have them check the existing radiator cooler very well - don't skip this, it's important.

This fix often works because the add-on cooler reduces the trans fluid temperature 10-20 degrees which is just enough to keep the PCS from calling for full pressure shifts.

Not saying it will always work but it often does the job and as a side benefit will greatly extend the trans and torque converter life.

The problem may not be mechanical.
The tranny might have it's own separate control unit, or the main ECU
may control it's shift points.
The ECU needs inputs from various sensors like throttle position, engine
load, vehicle speed and other sensor inputs to control vehicle
drivability. I would have the truck scanned for codes to see if
everything is working properly.
Otherwise the main ECU may need to be reprogrammed to correct the
problem.