Fresh 727 Street Strip Trans S.B. Wont Drive

Just noticed you said you have Yukon axles. A few yrs. ago a buddy of mine put a set in his A-body 8-3/4 and they seemed to bottom out but in reality they were just barely long enough to engage in inner set of side gears (engaged only about 1/4"). The first time he took it out and jumped on it the splines stripped off the end of the axles and it would no longer pull itself. When the rearend was jacked up the tires would turn, but not when on the ground. The driveshaft turned all the time.

I think I'd pull the axles out and have a look at the splines