Engine/Transmission? Shudder at exactly 55 mph is n 95 RMW

132k. Engine runs smooth in N or P. A little rough in D with foot on brake. Can feel a little skip or two when first accelerating then normal until 55 mph when there is a distinct shudder. Dont know if it’s engine or transmission or something else. Opti and WP recently replaced. Shudder was there before replacing opti. Otherwise car is great. Any help would be greatly appreciated. It’s a little annoying. After the 55 shudder everything smoothes out

You could have a bad fuel injector, a weak cylinder, bad plug/wire, or the harmonic damper rubber ring is worn out. Putting it in gear and feeling a shudder at idle tells me it's probably not the transmission.

Thanks for the response. You’re probably right about it not being transmission. I just got code 44 off the climate control. Funny you mention the bad plug wire. I once had one lose insulation from vibration against the block and shorting to block caused it to run rough.

The shudder at about 55 under light throttle is a notorious issue, and caused by the transmission. There is a product called anti-shudder at NAPA. It will stop the shudder, but your trans may be on its way out.

Now getting both code 13 and 44. Any ideas? Rough idle at stop seems to be getting worse. Once moving it still smoothes out.

Those codes are O2 sensor related. 44 is lean on left bank.

How is your fuel pressure?