4l80e harsh shifts - no codes/high line pressure

[1996GMC1500]

Hey guys. First post in 6 years as a member! I'm hoping a transmission guru can give me some good advice as to how best to proceed with this transmission.
'96 GMC 1500 6.5TD/4l80e with ~240,000 miles is shifting hard at all loads. No DTCs present, I've verified resistance of solenoids, and checked pressures to be:

Park, neutral, and forward gears - ~170 PSI
Reverse - ~270 PSI

These are at rest with foot on brake. Haven't stall tested or road tested. My understanding is that 171 PSI is max line pressure except for reverse. If this is the case, can I assume a mechanical cause with the lack of DTCs? What's most likely? Restriction? PC solenoid? Wondering if there's any other tests I could perform before I throw parts at it. Is it worth finding a shop with a Tech II that can step test the solenoids?

Any advice is much appreciated!

[Snowbound98]

I had a similar problem years ago. I too checked pressures and resistance of shift solenoids. I even went as far as replaced shift solenoids with no luck. My problem ended up being a throttle position sensor. But mine was on an older DB2 pump with a throttle cable. If you can get ahold of a scanner it's an easy test to watch voltage as you push on throttle.