BLD does nothing to transfer power from the rear wheels to front wheels (JK is effectively a 100% rear wheel drive, unless you pull on the lever and put it in 4H or 4L).
Select Trac is an AWD system that allows for on road use. I believe it's the same transfer case as in Liberty (KJ). The MP3022 transfer case uses an electronically controlled clutch pack to bias torque from 100% rear to 50/50% front/rear. The transfer case has no center differential, so power can only be supplied to the front axle when the rear wheels begin to slip.
It's kind of what WJ does but with viscous coupler thingy in the transfer case.
For JL this is an option on 4 door models, not on 2 door ones. I guess short wheelbase is too twitchy for it.
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'12 JK Rubicon V6 3.6L
'15 KL Trailhawk V6 3.2L
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