Try this:
I’m sure you have a road where you know you can make this happen all the time on, correct?
Well, next time you are driving on that road, shift the transmission down to 2, and hold the vehicle speed to the same as when it normally occurs. The engine will rev higher, but this is how we can eliminate the engine and transmission from being the cause.
If it still happens with the transmission set to 2, then its a frame/chassis related issue.
I would recommend taking it to a body shop, and have them look for crash related damage.
If it no longer happens when it is in 2, then you are looking at a transmission or engine related issue. Could be as simple as engine mount that went bad, or as complex as a transmission converter that is either failing, or is improperly programmed.
BC.