sand ladders seem quite expansive...
I bought some 4' long aluminum diamond patterned ramps from Inudstrial Metal Supply in San Diego for about $80 total and have used them before semisucesfully in mud and snow.
and by the way, these are 31" bfg mud-terrains looking more like cheese wheels
and just in case you are thinking what am I doing in the middle of some field tearing up the land? - we were following a dirt road and we got to a huge washed out rut that we couldn't cross, so we had to turn around, the road wasn't wide enough so I decided to drive out to the side of the road onto a plain looking green field to make a u-turn. well as soon as I entered the green field the truck just sank right into the super-soft mud. was one of these moments that reminds you that not everything is the way it looks...
now back to the topic. these boards are not anywhere strong enough for bridging though, but they double as my motorcycle ramps (if I can find a nice hill, cause they are somewhat short for the tall bed of my truck) and sometimes even as tortilla heating pan, although I heard aluminum is not good for your health