The difficulty with towing is stopping the trailer at highway speeds, the vehicle tow rating is based on this. You won't have any difficulties at slow speed. If you keep the wheel very straight, than you can pull straight forward in 4 wheel drive mode on the steep/slippery ramp when necessary. Go back to 2wd as soon as you can, and do not make any turns on pavement in 4wd mode or you could damage your driveline.
Edited to include Motorcharge's comment:
US tow capacity of the XJ: 5000 lbs
UK tow capacity of the XJ: 7100 lbs
If you drive your XJ from UK to the US somehow, you have to leave your trailer at the border...