I have the same problem with my '84 solid axle.
I mentioned it to gearman before I had him do my front
posi, hoping he would look at it, but I think he forgot about it when I took it in, so did I.
On the phone, he said it might the the 'trunion bearings'.
Tires are wearing sooner on the outside.  
Other possibility is a bent housing, but I don't know how
to check for that.  I don't wheel that hard, so that seems
unlikely in my case.