drive axle stays. if your measure question refers to the angle, simply put an angle finder on the top of the track(inside) just about where the rail caps are.
trying my best to relate this to you in understandable terms, so lets try this: if you have a grasp on the approach angle, picture the skid being moved back thus pulling the track back and changing the "bend" at the front of the track. yours now is running almost straight up(go back to the hand reference posted previously), you want it to be more "flat". picture a blanket draped between legs(blanket being your track, legs being your skid). you will have a big droop in the blanket(approach angle). now hold the blanket tight atop your legs(one leg being the drive shaft) and move your legs farther apart, effectively flattening out the blanket(approach angle). moving your legs apart is the equivalent of moving the skid back. hope that made sense, it was an off the cuff analogy.
the suspension doesnt care where its at in the skid as long as its original geometry is maintained, that being the distance between the mounting points. the distance back from the driveshaft is directly proportionate to track length and mounting points.
i would first experiment with relocating the skid back and ridding the rail extensions(you need to measure those to calculate the skid relocate). once you reconfigure the skid in the tunnel to alter the approach angle, from there its just basic math to get to 155.
i would have done that set up completely different. someone put alot of time and effort in the drop and roll(been there, done that a few times) and failed to take advantage of their efforts. but, in the long term, with the d&r, once you get all the bugs worked out, you will have lots of room for a 2.5" lug on a 155. without it you would be set on 2" max.
your sled will still suck as a mtn sled as its too nose heavy, too low in the front, and the ski stance is too wide, but all those negatives can be addressed, except the weight penalty(for the most part), with the addition of other oem parts.