What are these brackets for?

There are one on each side welded to the framerail - under the backseat with holes thru the floor to mount something from the inside... but what?

It's just where the framerails have the upswing from the passengers floor to the cargo floor. Doesn't seem to be a seatmount.. i can't figure it out.

I can't tell from that pic, maybe a jack mount?

I was thinking maybe the lower mount for the seatbelts? Mine never had a backseat or belts in it so i never unbolted it myself..

The jack mount is below the red circle in the pic. That is most likely the mount for the lower outside (male) end of the seatbelt. Depending on the year of the XJ the back belts were mounted at either 2 points or 3 points.