Before I bought an Olds rear for my '53 to put an end to blown rear ends I used to carry a spare "P" case posi rear end already set up in the trunk along with a pair of five ton jack stands and a floor jack.
After breaking the side gears into two or three pieces, I would swap out the rear end on the side of the road. Then go home and rebuild another one. After I swapped the Olds rear out for the weaker Chevy rear end I started to learn how to rebuild a Borg-Warner T-10 in an afternoon (usually ripped the second gear syncro teeth off of the gear, which could be due to my grinding off every other syncro tooth to aid in speed shifting). Trannies are much easier to replace on the side of the road, than a rear end.