Western Chassis rear Flip Kit or DJM Flip Kit
 

whice one set's a truck lower

I think WC does. I've got the DJM and it's 5", but I think Eric did the WC and it took him down 6".

Shouldn't be any difference. I only have experience with WC, but it has to do with the thickness of the leaf spring pack more than anything. I think anyway.

FYI:

Just noticed that JC Whitney is now selling Western Chassis suspension stuff. A 3/5 lowering kit is selling for $199.95 using a 3"drop coil up front and a flip kit for the rear. Don't like the idea of a drop coil. They list a 2.5 drop spindle and 4" rear shackle & hanger kit which runs $349. You may want to purchase from a bona-fide suspension vendor for support, like where Mike bought his stuff.