Any grinding noise? Try stopping before attempting reverse... ?? just kidding!
Comedy isn't my strong point.
If the soap trick doesn't work (which it should!), I'd consider attempting the shift under hood, with and without the linkage hooked up to column shifter. This would be attempt to isolate the binding culprit. The more worn out the linkage, the more unforgiving of sloppy shifts to the point where it may simply refuse the desired shift entirely.
