Rear Diffy Lowering

[Kneale Brownson]

I'm getting ready to lower the rear diffy to allow replacement of my
leaking fuel tank.  Mr. Bentley advises "lock the  differential" so I can
take out the bolts, I guess.  Earlier he suggested I disconnect the battery
before disconnecting all the stuff at the top of the tank.   Is there a
manual locking for the rear diffy?  If not, do I really need to lock it to
take out the bolts?    If I need to lock it, do I reattach the battery and
operate the switch in the console to lock it?

After I slide the halfshafts away from the diffy and "tie 'em up" somehow,
will I need some sort of gasket when I go to reassemble things?  I can do
this pulling out of the halfshafts without disengaging their outer ends
from the hub/suspension?

Anybody got any idea how far down I have to let the diffy sag in order to
get the tank out?  I want to figure out how tall a support I need to keep
the diffy from dropping too far.