Lifting the body off the frame of my 89 Trooper?

Used cars go figuer! My prodject is an 89 Trooper. I bought it for 500$ and maybe now I know why. I removed the carpet from the back and where the wheelwells attach to the floor on both sides was rusted right through. I want to weld it back together and water and rust proof it, but the frame is in the way on the bottom. I was wondering if I need to lift the whole body to get at these areas or if I can disconect and lift just a portion of the body temporarly to get at the problem areas. Also the support beams have a fine sandy substance inside, is this suppoed to be there or is it foriegne?