Tony,
I just ordered a front Harrop e-locker today for mine (part No. A11525). It's costing me $1650 delivered to Mt Isa, so not cheap. I will be installing it myself and intend to do an install thread with plenty of pics soon.
The reason that I was interested in the LOKKA, was because I was toying with the idea of buying a second hand D40 front diff out of a 2.5 auto (so therefore a 3.54:1 ratio front diff) that I could get for $300. Since the carrier out of the 2.5 is a 2 pinion and is the same dimensions as my 3.133:1 diff (I think), I could have installed the carrier from that, to my gears and fitted a LOKKA for $600 which would mean it would have cost me $900 all up. (keeping my std 3.133:1 gears inside). The 550 and STX with the V6 engine have a 4 pinion carrier and a LOKKA is not available for them.
Both Harrop and ARB state 2 different design diff locks for the D40. One suits 3.69 and up gears and the other for 3.54 and down. That says to me that their are 2 different carrier sizes to cater for the different sized crown wheels, which makes sense. Their is probably some people on here who have fitted either an e-locker or air locker to their auto D40, that have their old, open center carrier lying around in their shed. Wouldn't mind measuring one up and comparing to the 550 4 pinion carrier.
Anyway cost aside, I reckon the e-locker will be great. Not looking forward to the install but am looking forward to the playing.!
_