Transmission struggling

Are you towing with cruise control engaged?

Can you describe "Upshifting very hard"? Is it a jerking motion or something else?

Older? What year? What trans?
First order of business would be a fluid and filter change. I seriously doubt this will fix your problem, but for a hundred dollars probably worth it to try.
If this does not fix the problem, then it is a transmission rebuild.

We had a sensor go out on our last truck (ford) and it did something similar. I forget the part but it sensed engine and transmission RPM and adjusted to make the shift smoother. Changed out the sensor and all was good.

Depending on what year it could be lots of things that are almost impossopible to troubleshoot over the Internet.
What is "older"?

Maybe.....throttle valve cable, pump pressure regulation too high, broken accumulator spring, check ball wore hole through valve body plate, valve body sticking, vac line to accumulator leaking, etc.

It is a 2000 GMC 2500 Sierra with 119k miles. Only had it a year but original owner pulled pretty large Airstream- definitely bigger than our 27 footer
The hard shift is just that, it builds up RPM and then up shifts (not in cruise control) with a bang and jerk to next gear. Again, this is on flat land after quite a bit of down hill.

Engine is 6000 Vortec (364 CID)
4 speed Auto Electronic Trans w/Overdrive w trans cooling system

I had it into the Transmission shop recently and he could not find anything wrong (of course it wasn't towing anything) but he did perform the service.

Delayed upshift can often be attributed to engine performance but diagnosed as transmission issues. Doesn't really sound like your symptoms but delayed upshift can be much harsher than normal when they finally happen. Just throwing that out there.

If it is a 4l80e, I am surprised it has that many miles on it doing towing duty. I usually only get 30K with 5-8K mile fluid, and every other change a full flush of the TC.

While a lot of folks think that trans is a great towing trans, it is in reality, one of the worst ones out there. It has the tallest first gear of an auto trans, so it really has to work to get a load going.

Marty

Have you checked for any stored OBD2 codes? Apparently there are a few which can cause the transmission to default to high line pressure (i.e. hard shifts) to save itself.

One example is a slipping torque converter clutch. (like this guy)

My trans issues were caused by a corroding engine computer. No codes thrown just super high trans temps to full trans failure, tear down, new torque converter, and stage 3 shift kit to figure out the out the PCU failure.