1,400-1,500 RPM's shudder like rumble strips

It has the wrong transmission fluid in it. The shudder will only occur once the engine is warmed up, and around 30 to 40 mph. You can verify this when the shudder is occurring by holding road speed and the accelerator pedal perfectly steady, then lightly tapping the brake pedal with your left foot.

The shudder is caused by the transmission's lock-up torque converter locking too aggressively, then engine torque breaks it free, then it locks up again. This occurs at the engine and road speeds you've observed because conditions call for partial lock-up to come in gradually, but the transmission fluid doesn't have the right additives to allow that to happen.

Tapping the brake pedal makes the computer think you're preparing to come to a stop. That's one of multiple things that tells the computer the torque converter needs to unlock. It will relock two or three seconds after you release the brake pedal.

Keep me posted on your progress. My daily driver is a '94 Grand Caravan with the same shudder. It didn't get any maitenance service to anything from the previous owner except multiple repair attempts at solving an intermittent failure to up-shift. That occurred for me about two times per month for the last two years, and it was never the dreaded "limp mode" where you have to turn the ignition switch off, then restart the engine to get it out of that mode. This turned out to be all related to a very unusual sticking throttle position sensor. I finally found it by accident when it developed a crank / no-start issue. The sticking sensor put the Engine Computer in "clear flood" mode where it stops firing the injectors. It also intermittently caused the Transmission Computer to think it was under hard acceleration, thus the need to wait until higher speeds to up-shift.

The point is, the van was in the shop at least three times with the previous owner, trying to solve the up-shift problem, and I'm sure at some point they put the wrong fluid in it.