Steve, I don't know what trolly jack you have. but if it sounds like its not suitable.
A Wolf says, the best place to use a trolly-jack on an xj-s is either the front subframe or the plate under the diff, protected by a decent piece of timber both to spread the load and to protect the car.
The main trouble I've had with axle-stands is they really *are* designed to support an axle which for an xj-s is useless of course. The photo below shows a set of mine to which I have welded a 'ring' (3/4" steel pipe I think) which nicely supports the Jag jacking points without any possibility of the stand moving or slipping.
http://i11.photobucket.com/albums/a183/ ... 9Small.jpg
While the rear is on the stands, remember that the only things holding the rear cage and suspension is four perished rubber mounts, take care; Having had a Land Rover fall off a scissor jack many years ago while my legs were underneath has made me very carefull since! By luck, the brake drum hit the floor, not me
Pop