Are sure your real axle is in good shape? My '85 Dodge had a posi-trac that did the same thing, it would change lanes when stepping down or letting off. I found the axle splines were worn where the side gears meshed. The side gears were a harded material than the axles. It ate the right axle faster than the left which made it pull to the right on take off, and jump left when letting off. After a swap to standard axle it drove as good as new!

Don't give up on the straight axle yet. Make sure everything is in good working condition before commiting to drastic work. JOE