Mustang Differential

The clutches in your posi are worn out, resulting in the afformentioned problem. Rebuild kits only run about $50; shouldn't cost too much to have done at a reputable place. If you don't have gears, now would be a good time to consider having some installed.

the reason it may only be spinning one tire in first, is because of a gradual increase in power, as opposed to more than likely you're "dumping" into second (quick release on clutch) causing more shock and who knows, i know it usually helped me get 'posi' as we call it

but soon 2nd gear won't be 'posi' and it'll be peg legging too...

its an easy fix, you need to order a traction lok rebuild kit (fords term) costs about 50 bucks, pretty easy to do if you're mechanically inclined at all.

i ordered mine from D&D for 46 bucks i think, and got shims, spline plates (steels), clutches, and then friction modifier, and a new pinion pin bolt thingy too.....pre- lock-tited!!!!

yeah, the stock differential was meant to make both tires spin like you want, except its designed to slip turning and such (without spin)-- yours is just worn out.
get that puppy rebuild, and crank some donuts (its easy to rebuild i and many other stangnet guys have done it)

p.s., make sure to shim it properly! don't wanna drop a C clip do ya?