Hi all,

I have a 2012 GLK 350 4Matic ~150k miles.
First of all, unfortunately I am not very good with cars.

My car has a gentle vibration when shifting gears from 2nd to 3rd at about ~1500 RPMs. You could feel it at low speeds when accelerating from stationary. Like when you are stopped on a red light and you start to accelerate it shifts fine from 1st to 2nd and then from 2nd to 3rd you can feel the vibration.
It is not felt in the steering wheel, its felt on the car floor. When the vibration is felt, if I accelerate more and rev it up to 2000 RPMs it goes away immediately.
It doesn't do it if I manually choose to shift gears. It only does it when on automatic.

Always did maintenance on time. Changed oil regularly.
Sent it for a control and was told the tires might be a problem. Changed them, looked like it stopped for 2-3 days then it came back.
Then I was told I should check to do a transmission maintenance. Sent it to a transmission-repair shop and was told that the transmission was perfectly fine. It shifts perfectly they said. Did a transmission maintenance, flushed and changed the fluid. Asked if there was any other thing the transmission needed and they said it was super OK.
Also they saw that one of the engine mounts wasn't on perfect condition. Changed that too.
Vibration stopped for like 2 days and seemed smooth, then the vibration came back again.
I asked some people that said change the other engine mount, but the engine doesn't shake at all and it doesn't have strange noises. Even when I am stationary and put the car on D with the brake held and put my other foot on acceleration, the engine doesn't give even the slightest sign of vibrating.
Saw online those videos about resetting TCM but my car doesn't have a kickdown switch button under the pedal.

Any ideas would be helpful. THANKS!

Have ujoints - cv joints checked . I ruined the u joint on the front driveshaft . You don’t want to have these break. Very expensive damage will happen.

Definitely get the front drive shaft from the transfer case checked for any play. @Mmr1 ’s comments are an understatement.
Other possible components would be:
Transmission mount - often responsible for minor transient vibrations;
Remaining engine mount - if one was bad, the other is probably not far behind;
Rear drive shaft attachments to transmission and rear differential;
All drive axles and CV joints.

Met a guy at a gas pump this morning. He also had the second generation 3.5L Gasser with 4Matic. Same color as mine and looked like new. Said hello to him and he asked me if I had had any problems with mine. I said no and he told me the driveshaft went on his. Did some damage, of course, including taking out the catalytic converter. About $4000.00 in repairs. I asked how many miles he had on it and it was fairly high. I think around 200,000. I didn't think to ask how many miles were on it when it failed. Both of us bought our GLKs new but mine has many, many miles less. In any event, a good reminder to check or get the driveline checked on occasion and especially as the miles/kilometers start creeping up.

Do you drive in E mode (green E) or sport mode (red R ) on dash. In E it starts in second gear not first gear. It could be a weak transmission. How old is oil and filter ? Do you feel any clunk noise when coasting and accelerating, or shifting from reverse to forward ? That’s the driveshaft u joint gone bad. I ruined my u Joint being stuck in snow slamming it from forward to reverse. Maybe motor mount ?