Bob,
It's interesting to me that you say that an open differential keeps a car straighter under heavy acceleration. Now I'll agree that on wet or slick (snow/ice) pavement a locked rear can tend to swing the rear if too much power is applied, but even on slick pavement you get better traction with a healthy posi or TruTrac. On dry pavement the only reason I can see for the car staying straighter with an open differential is that you're over powering the single drive wheel, and basically giving up any traction that you might have with the posi. In short, you're not putting enough power on the ground to push it crooked. A good unit applying equal power to both rear tires is by design going to push straight. Give me a Good Detroit TruTrac, or next best a healthy posi with good clutches any day over an open differential. That's one of those lessons learned many, many years ago, the first time going up against a posi with open. ;):):)