Well that info helps a bit. Now I would go more with a tranny problem, or a wheel issue. That said, IMHO ppl should clean the TB about every two years because that is all it takes to never have any issue with it.

Trannys I’m not knowledgeable with, but it I would put it high on the suspect list. The wheels should be easy to check. Does the low end shuddering repeat at a higher speed? I’ve had wheel issues before and when they hit resonances they will shake in narrow mph ranges. And other mphs the car rides pretty smooth.