no you shouldnt need to take off the axle or cut the floor. all you have to do is undo the top links, brake lines; axle breather and back box off the exhaust. then cut the crossmember as close to the chassis as possible. take the old crossmember to a local fab shop and have them bend one out of some thick wall tube (4 maybe 5mm)and the same diameter as the old crossmember. weld on the top link mounts to the new crossmember using the old crossmember for reference, weld on the brakeline and breather mounts and a new exhaust hanger and then weld it to the chassis. drill a few holes in the crossmember so you can get in with the waxoyle and jobs a gooden.

if you mean the rear most crossmember its a tank guard and tank off job but you wont have to have any fancy bends just a straight peice of tube or even box section will do.

if your in the northwest i would come give you a hand.