what exactly does a transfer case drop kit do for the truck...my 1980 custom has a 4 inch lift and id like to go to 6"...it recomends using this transfer drop kit....what else should i know before putting this 6" lift on my truck ???  thanks guys  

It lowers the transfer case cross member.  It will reduce the rear drive shaft angle, and will reduce the front drive shaft angle at the axle but it will increase the angle of the CV u-joints by the transfer case.  Lower the transfer case cross member too much and it puts extra stress on the engine mounts.  I have used washers in the past, or you can use nuts that are larger than the bolts mounting the cross member but I would limit the drop to about 1" max.  I went much less than that even.

You would be better off using a double CV front drive shaft rather than lowering the transfer case and related.  That way you wouldn't have to rotate the axle tubes in the differential housing to reduce the angle on the U-joint at the differential (the caster angle has to be kept in at certain degree for proper handling).  A 6" lift is the border line of having to rotate the tubes, but I would go for the double CV shaft instead.