How to check differential locks?

[SJ]

Car is 88 5kq.
Lock engagement is with knob. "1" selects the front, "2" selects front and
back.


In the past, I recall that when the diff locks were engaged, you could hear
and feel the tires scrubbing as you went around sharp turns. I dont detect
that now.

Yesterday, I was checking the diff locks. Drive slow and engage the locks. I
did not feel or hear anything happening.
Today, during normal driving, I hear a whine. Its not engine dependent. The
whine comes and goes, but seems to kick in at a certain MPH.
Maybe this is a result of the cold temperature. It got really cold
overnight.

Is there a way to check the differential locks with the car up on jacks? I
dont mean a check to see if the lever is moving. Is there a check at the
wheels to see if the differential is indeed locked up?

I know if you turn one wheel, the other wheel turns in the opposite
direction. Is this still true for locked differentials?

When the diff lock lever engages, does it engage directly? or do the
internal gears need to move a bit and get into alignment before the lock
lever drops into position and the diff lock is engaged?
Someone mentioned that it takes a bit before the locks engage. I suspect the
delayed engagement is a vacuum related problem at the actuator.

Do the diff lock levers freeze up with age? Like the rear E-brake lever?

Is it possible for the diff locks to be partially engaged and produce that
whine?

SJ
85 Dodge PU, D-250, 318, auto
85 Audi 4k - - sold but still on the road
88 Audi 5kq
90 Audi 100q