Be very wary where you put the jack if you need to jack a certain wheel put the jack under the axle tubes on the axle ( do not jack on the centre diff casing, especially a split axle design they are not very strong ) just below the leaf spring position it has always worked for me. Also don't forget the hand brake on a GMC works on a band brake on the output of the transfer box which is connected to the rear prop shafts, so if you have one wheel on each rear axle off the ground and the truck is situated on a bit of a slope it will move off down the hill, only jack on the flat if at all possible, oh and bear in mind axle wind up as the rear two axles on the GMC are locked together both diffs have to go round together, so you can get wind up where the axles are fighting each other, when you come to jack any of the rear wheels as it comes clear of the ground it can spin a bit, probably only a couple of inches but under quite a lot of force, if you are not expecting it, it can be quite a surprise.