what about a standard posi unit like a factory trak-lok... it would slip enough to allow you drive under low power manuevering in the pits, but will lock up when you put the power to it. its not as good as a 100% locker, but it can be done for about $350 or less and will work better than an open diff. i think when i get the money to re-gear my truck i am gonna spring for a factory D60 posi unit out of the rear of a half ton on something, i've heard good things about them.
i re-stacked the clutches in my D80 rear posi last year and it was like a spool. i spun both tires all the time, even at the big end of the track when the sled stopped me...it kept the rear end locked up.
