Used a 5/8" (I think thats what it was) box end wrench to hold the nut by cutting the wrench in half and slipping it into the cavity where the nut resides from the top side. Going in through the opening that the steering linkage passes by. Then to turn the mounts I used a chisel and a hammer after cutting away all the rubber so I'd have a solid metal surface to hit.
But like Hardass said (I learned this after I cut my wrench in half, haha), if the belly pan is off the bottom of the bulkhead, there are little wells (they're kinda deep) that you can put a socket up into that give you access to the motor mount nuts. I don't remember terribly well, but they may be a bit hard to locate. Just look directly below the mount I suppose. I would go that route, and first try a channel lock on the mount before cutting away the rubber (that sucked to do). The mount should have little tabs that stick out on the base that you could use a chisel on, unless the mount is in upside down (mine was) then you'll have to make a notch or some deformation on the mount base to use the chisel on.
I did this on an 04' about a year ago in the back of the truck and may have a few things backwards from memory, if you need clarification just post up the next ?
good luck