Engagement speed  shouldn't be any different now then when it had the
auto hubs.  You can push the button at any speed up to 40 is what I
think the manual says.  You can do it even if you don't have the hubs
locked but you won't have 4 wheel drive, just spinning shafts!!  With
the hubs locked you can probably engage it at any speed actually,
since with them locked all the parts are already spinning at
engagement speed but I wouldn't do it because you just never
know....I'd stick to the 40 mph limit.