vehicle has a vibration between 20 and 25 mile an hour. seems to be in rear

What is your question?

I took to dealer twice and they could not find problem. Had two mechanics drive it and they say seems to be in the rear end of car. One of the mechanics was my transmission mechanic and he said it was not the transmission. the other mechanic check the drive shaft and found nothing wrong. but he seems to think it may be in the differential. He suggested to take it back to the dealer when i took it back previously it was under warranty.

What makes this problem better or worse?

no

How long has your 2015 Cadillac SRX had this problem?

a year