rear end feels loose
 

If I take off fast, or let the 1-2 shift happen near WOT, the rear feels "loose". Tires arent breaking traction, but the rear almost feels soft like it is loosly connected. Planning on a re build this winter. What should I look for? I dont really want to have sub frame connectors hanging out under the car, though.

Good thing this forum is family orientated.
try posting this in clubhouse and see what reaction you get.

If its a cone or clutch type posi it could be worn out.
Jack the car up with one wheel still on ground,and raise the other off ground,see if you can turn the wheel by hand.
Note this is for posi only and make sure its supported and blocked for safety.

I know i have an open unit (car off the ground, rotate one wheel foward by hand and the other rotates backward).

However, I can lay down 2 black pathces sometimes??

How sure are you that the tires are not loosing traction? Just because they dont leave a striking black line or the typical noises doesn't mean they aren't spinning. There is a wide range of what kind of traction you get with different tires and their age plays a big factor as well. But if they are not spinning take a really good look at where the rear mounts onto the leaf springs. Look for any signs that the rear is moving around on the springs and the condition of the rubber perch pads. An upgrade to the stock configuration is to use 2 sets of u-bolts on each perch instead of 1 set u-bolts and one set of t-bolts.
As far as a rebuild, in order of priority, change the fluid, replace leaking seals, replace axle bearings, replace all the seals. Thats about it unless it's making noise.

Looks like you have a 68 FB in the pic. I had a similar feeling once and the lower shock mount was tearing from the bracket. I have a 67 and I think the brackets are different, but check them out. Also make sure the leaf spring perches are tight to the underneath. Look for any shiny spots around the bolts and washers, if so it's moving.

check shock bushings too.
a non posi can spin both wheels if all conditions are right