Mark
1500 kg is on the heavy side for an x trail, but should tow without snaking.
I would only expect power issues, hill starts, long inclines etc.
Ensure you have a lest 85kg nose weight, I find 90 kg ok but find the front end feels light over this, just my preference, not saying you cant load to 100kg.
Also I always increase the tyre pressures to full load pressures and slightly above for the front. That is 36psi rear, 34psi front.
I always use an antisnake device I find the alko2500 to be very good.
Your problem could be the van frontage that is maybe it has a poor aerodynamic that's a bad partner for the x trail?
Finally if weather permits I always put the sunroof on tilt, I haven't a clue if this helps with airflow, but it certainly tows my bailey senator Vermont ok. (1330kg approx).
Ps try towing at 55mph rather than 60mph. Side winds, road surfaces etc all contribute to stability.