Twisty,
The reason you can't tell on your truck is because you've lowered the front bashplate to cover them.
Without the front bashplate on they look like this -
You can see how much more exposed the front diff mounts now are with them being dropped another inch lower......the factory bashplate protects nothing....its like tin foil for offroading.
This reduces your 3" worth of clearance to the same as 2" without the 1" diffblocks installed under the front diff.
The diff drop because of its "rolling forward" of the front diff when fitted also places more stress on the rear diff mount and changes the Universal joint angle to the front diff, thou this shouldn't be by much anyway.
Tacoma's and FJ Cruisers in the US have been running 35's with 3" lifts , no diff drops and crawler boxes for quite a few years now without any problems.
Heres a diagram explaining what I mean in regards to the 1" diff drop only giving you at best 1/2 an inch reprieve at the CV shafts -
It was like a cantilever effect and the CV shafts are halfway between the rear and front mounts.
Hope that helps abit.