Recently installed new rear axle 3.73 10 bolt w/ auburn pro posi unit. I've re tighten axle U bolts several times, last time I couldn't tighten them anymore cause there as tight as there going to get. The axle on the driver side keeps shifting forward approx 1/2". How do I get it to stay put? Spring pads are all new also. I do have a set of slapper bars that I haven't installed. Would they help any?

 

There is a locating pin, which is part of the bolt that sandwiches the spring plates together, that fits up into the spring perch on the axle. There's no way the axle could shift forward, unless that pin isn't in, or if there is that much of a gap up top where the pin is dropped down out of the perch. Can't see that part in your pics.

 

It's not at the top,its at the bottom plate that has the 3/4" hole in it. The nut that holds all the leafs together moves out of that locating hole.

 

Couple things that could cause the nut to get out of the hole. First the rubber insert sandwiched into the spring perch might be too thick, and not letting the nut sit deep enough, regardless of how tight you get the U bolts. Second, the nut may not be the correct outside diameter to fully fill the 3/4" hole. They're all the same thread size, and pitch, but there's a couple wrench sizes for the nuts.
On the originals, one side had a U bolt, and the other was a pair of T bolts. That didn't hold the axle from spinning under torque as well, but did a better job of keeping it from slipping forward/back under torque. You might also consider putting nuts on the inside U bolts on the top side, so once you have it all torqued down, you can run the nut down against the perch on top to lock it up. If the perch doesn't tighten against the plate, put a couple spacer bushings in between to ensure it's all locked up tight when you tighten it down.