Re: 27a differential ??
With the front axle on jack stand, put the T/C in 4WD (to lock the front drive shaft). lock the hubs in. Alternately try to turn each front wheel. The other wheel should turn in the opposite direction. If the other wheel does not spin, have someone else observe the movement of the side gears inside the pumkin, this will let you know wich side is broke. I will put my money on a locking hub that is: not locked, broke, or improperly assembled.
good luck, jjc