'Thump' went going from 1st to 2nd

I have a 2001 nissan frontier 2 wheel drive pickup, automatic. 100K. When it goes from 1st to 2nd there is a loud thump and the car jerks. Hard to say but it seems worse when the car is ‘cold’. It happens in other gear changes but not nearly as bad.

What can you tell us about the maintenance record of your transmission?
If your answer is “none”, then I predict that the future for that trans is not good.

Yep, I have little to go on. Purchased it used @ around 85k and in 4-5 years now have only put 15k on the vehicle.

If you have no idea of what the prior owner(s) did regarding trans maintenance, and if you have not done any trans maintenance in the past 4-5 years, I have to say that I am not optimistic about your transmission.

Start researching independent trans shops in your area, in order to decide which of them to take your truck to for diagnosis.

Whatever you do, DO NOT take the truck to AAMCO, Lee Myles, Cottman, Mr. Transmission, or any other chain operation, unless you want to be told that you need a new transmission–whether you really do or not.