I think the biggest reason the ZF6 isnt very popular is mostly to do with cost. The 5 isnt cheap and the 6 is alot more last time i checked. Also like said above, all you really gain is a granny gear, which you dont really need anyway. Other reason might be that it is physically much bigger, needing firewall and/or tunnel surgery, custom bellhousing adapter at the top, requires a newer t-case, which would need a new driveline, then you have to deal with the clutch. Just alot of $$$ if's and no why