I had the cover off my stock 10 bolt with 2.73s.  i saw spider gears, but no posi carrier that  thought of.  yet, i can do a burnout and leave 2 dark black marks.  usually the right side is darker, and i will have more rubber on the right rear 1/4 panel than the left.  But, there are 2 definite black marks.  What gives?  

as long as both wheels are getting good traction they will both leave skid marks, normally a open rear only the wheel with the most traction will leave a skid mark, but if you manage to find a spot where their both equal or pretty close they both can spin.  we live on a dirt road and i could always get my old camaro to do it, with the open rear, thou i think that was more a case of neither was getting good traction.  

I've seen open diffs where the cross-pin and spider gears were so worn that they would actually bind up under hard acceleration and lock the diff like a spool. Try pulling the pin out and see if it's worn. When it gets like this, bad doo-doo is not far away!
Steve  

Probably is worn.  Did a burnout (now that i got that nasty mis fixed, Steve) and went to back up to see the marks... so juvenile!  I heard alot of crunching going on.  Might be time for a new rear

Can I get a posi unit for the stock 8.2 housing?  looking for between 3.55s, and 3.08s.