how is a stock 86 iroc posi disk rear end worth with 70k on it?

Depends on the gear ratio... from nothing to about $200.

stock 3.23's? probably pick it up at a junkyard for like 80 bucks, you probably wont want a rear end any older than 100k posi would probably be all worn out

my rear has 70k on it

An '86 would have the cast iron saginaw disc brakes which weren't much better than the drums and require a recall kit to fix some known issues with the parking brake/automatic adjustment. They're not very desirable.

Repair/replacement parts for the posi unit aren't available either. If it's a two series carrier (2.73 or 3.08) the posi unit is even weaker than the 'performance' carrier in the 3 series.