Feels like slipping, bucking, or jerking when driving about 40 mph. No check engine light. Not felt below or above that speed range.

What is your question?

Purchased used, and within several months, differential went out. This "jerking" has occurred for the several years that I have owned the car. Issue was minor at first, but more noticeable now. Transmission shop says the trans is ok, but check engine light indicated a faulty timing solenoid. Replaced both. but no improvement. Apparently unrelated to the 40 mph jerking. Can't find info where others have had this to occur.

What makes this problem better or worse?

Nothing. Is present at all times when driving near 40 mph

How long has your 2007 Kia Sorento had this problem?

Several years