This is an issues that's been going on for two years, has gotten slightly better but still happens.

The transmission will go into what I'm guessing is "high pressure mode" where it slams into R/D from P/N 1-2 and 2-3 shifts a very harsh.
It used to only do this after WOT shifts up/down now it just kind of does it....whenever

Also at highway speeds if I manually downshift to 3rd to pass someone and then shift back in it 4th it hangs in 3rd, but will eventually shift back after 10-15 seconds.

I changed the TPS that was actually broken with no noticeable change.
I'm using Valvoline maxlife synthetic fluid.
I have NOT had the pan off.
Running stock silverado 1500 convertor (as far as I know)
B&M stacked plate cooler.

Like I said it's been doing it for 2 years and has gotten less frequent but I'd just like to know if it's something easy I can fix or if it should just wait until I get a new transmission.
Scantool/torque says my shifts are taking about .5xx seconds with a 0.1xx-2xx error under normal operation.

 

Have you scanned for engine and trans codes?
Various engine codes will put the trans into "high pressure mode" and give you very harsh shifts.

 

If it is throwing any codes, my Scantool SX isn't picking it up.
Only codes it's ever had was:
the coolant temp sender came unplugged
O2 sensor died and the knock sensors died.
All of those were fixed when the light came on.