If you decrease the throttle just BEFORE the shift, or just after it happens, if it is the torque converter clutch causing the issue, the issue will markedly decrease. It will not disappear, just decrease in intensity.
On the other side of things, if you increase the throttle at the shift point, the intensity of the vibration/shudder will increase, too.
Now, in the earlier years of Chrysler using lock-up torque converters, the initial lock-up speeds was like 27mph or when the 2-3 shift happened. To me, when the shift happened so soon, that meant that all of the acceleration (unless there was enough throttle input to trigger a part-throttle downshift into 2nd gear, all of the acceleration was in locked-up third gear, which would be slower than expected with less engine sounds.
But I observed that Chrysler used more-early shift points anyway. Their orientation toward earlier shift points also tended to coincide with the way exhaust emissions were measured, as in "Grams/Mile" rather than the earlier "Parts Per Million". Which also coincided with fewer rear axle ratio choices compared to what could have happened in the earlier model years of the 1960s. AND those choices were usually more "highway friendly" in lower cruise rpms, too.
By the 1980 model year, the torque converter lock-up point had been raised to about 53mph, which tended to work better and be less-intrusive. When the lock-up happened, the bulk of any acceleration's need was minimized and the engine was producing more torque anyway. Just worked better. For ANY engine which was spec's for "HD Service", as in police vehicles and trailer tow optioned vehicles, no lock-up torque converter.
Changing the lock-up speed can require taking things apart, as the torque converter control is mechanical rather than electronic, internally. Not easy to do. Seems like there is a separate little valve body for the lock-up function? Taking it apart, a stiffer spring might be needed to delay lock-up?
When my '80 Newport started to shudder when the converter locked-up, I knew what it was and sought to minimize it with driving behavior modification. When it locked-up and started to shudder, I would back-out of the throttle enough so that there was NO or decreased torque load on the converter clutch. After locked-up, then I'd ease back into the throttle and continue onward. EVEN if I was on an incline and was trying to maintain speed! I knew what the eventual "fix" would be . . . a new lock-up torque converter. ATF was the recommended Dexron-type atf and had been changed, too.
ALL of the OEMs had shudder issues, mainly due to normal wear of the converter clutches. They also upgraded their recommended atf from what it had been, "friction-modified" as they termed it, to better tolerate the initial slippage of the converter clutch when it locked-up. For Chrysler, it was the ATF+ fluids, from the prior Dexron-style fluids with a friction-modifier additive, in about 1996, after the introduction of the LH cars.
In some respects, the better-spec atf might be considered a Band-Aid fix of sorts, BUT it can't hurt to try it anyway. It might "buy time" to save money for the eventual transmission removal and new converter repairs. IF it works well, so much the better. In the mean time, reducing the input torque to the converter when it locks-up needs to be tried, so minimize slippage and shuddering.
Just my experiences and observations,
CBODY67
