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Pete,4x4s do not have posi traditionally. Posi is a dangerous option to deal with during the winter on icy roads. 4x4 trucks with wide tires are even more prone to sliding and posi makes it worse. Now if your serious about getting off road, then posi is what you need, in the back. I found they're a little hard to steer with a locker up front. To each his own.
Good Luck, Have Fun,
KingFisher
"Posi" is short for Posi-Traction which was used only by GM. It was a limited slip unit, but somehow the name has been turned generic reffering to just about any type of traction adding device used in all brands of vehicles.
Every 73-79 F250 axle I've opened has Trak-Loc. From my experiences at the mudholes in my 78 F250 4x4, the trak-loc set up works get. I get true 4 wheel drive, not a common 3 wheel peel! I personally like the trak-loc, not bad for factory equiptment!!