I have found that insatlling the axle over the coupler first makes the job easier.
If your eyes are good. Just a small thin blade screwdriver stuck between the wheel face and the truck sideframe. Ease the frame away from the wheel face and slip the axle tip/point into the sideframe.
The first picture removing the wheel set. Look where the screwdriver tip is. Pushing the axle tip/point out.
The second picture replacing the wheel set. Look where the screwdriver tip is. Pushing the axle tip/point in.
The wheels/axle are not destorted/twisted, it just the why the picture seems to look. A bright light and a magnifying lens would help even more.
After the axle tip/point is back in place be sure the wheel set will spin freely.
If you have one of those foam cradles to hold the car, that would be a big help.
Either truck mount or body mount couplers, It would be the same putting the wheel set on or off the truck.
does anyone have tip for re-installing n scale wheels to trucks? fiendishly dfficult!