I'll take a different point of view...starting from the point that all of my 4-wheeling, much of it in the edges of spruce bogs in AK, begins with chaining up the 4WD pickup or the SxS.
- Chains will help, particularly if you do have a hard bottom and are only dealing with slick grass.
- What will help more is, if possible, lifting the trailer tires high enough to get them on boards: can you jack up each axle so that you can lay down a 2x6x6' under the tires so that you're not having to lift/pull the trailer out of the mud?
- Maybe use a bottle jack on top of 1'x1'x2" pads (stacked two deep) to get each trailer tire up and on a firm surface?
Once you're moving, don't stop until the Excursion and the trailer (ideally both) are on a hard surface