I can testify from experience that the M135 axles are not as tough as the Rockwells in the M35 series trucks. I have had pinion bearings go out in the M135 axles, and the axle shafts will twist off easier. We tried an expreiment with a center punch on the axles. The M135 axle shafts could be center punch marked with the punch; the M35 axle flattened the end of the punch with no dent in the axle shaft. The M135 differentials have ball bearings on the pinion, which proved to be the weak link. The Rockwells have all Timken bearings. I was using the M135 to haul grain, etc. on the farm. The axle shaft twisted off when we were loaded, coming out of a filed in muddy conditions, hitting gears hard as the automatic in these trucks normally do, especially from second to third in low range.
The problem with the Rockwells is the higher pinion location, which might be a problem in your Pig. You probably would not have any problems with the GMC axles if you keep good oil in them and check them out well before installing to be sure they are in good condition. The failed pinion bearings might have had some rust in the races I did not see.
Regards Marti