I also have 94 4.0 and have the same symptoms. Read somewhere that the cure is to lube the splines from the trans to the shaft. SOmething about it sticks due to dirt and crud build up.

The guy wrote that ford has a special grease for the spline and that it cured his problem.

Like I said this is something I had read else where, I don't have time to try and lube the splines or anything else, but if one of you guys try it and find that it works,let us all know.