Build my own posi ! Its no Detroit Locker but for $55.00 worth of two 8620 steel 5/8" thick blocks & 4 Ford Posi springs, a couple of drill bits & a drill press will make the parts the ricers & motorhome people are selling ! You will need to grind the humps flat on the big side gears for the blocks to ride on this only works on the 1968 to 1978 differentials ( the 1966-1967 diff. is completely different ) they have spider gears. Sorry to say they are only a 2 pinon spider gear set-up not a stronger 4 pinon ! You will need to make the case opening bigger on each side, so the block set-up will slide in between the side gears, hook up the little spider gears & pin. then pull out the little clips & pins holding the blocks together for assembly that is all that is putting pressure on the side gears ! Like I said its no Detroit Locker ! I see now someone is building a better posi but they are making gears & stuff at a machine shop ! I've got a 66-67 set-up it has a sun gear set up that uses 31 spline axle, I may weld up to make it a spool, but for the street its a pain to drive ! The 1968-78 are 33 spline axles anyway cant beat that ! Good Luck !

 

Once again, supply and demand rears it's ugly head. There are no other options for posi in the TH425 carrier, which is why the GMC folks can charge what they do. The factory never made one and no RWD carrier fits. You'll spend way more than $900 to custom machine something.