Mark relevant text. Then press:
Place cursor at beginning. Type the following (with returns at the relevant places)
A B C D E
This procedure cleans first the trailing whitespace per line, then, per line end inserts A, B, etc. one by one at these end of lines. Other replacement patterns, are, of course, possible. It is not strictly a rectangle transpose, but quite versatile and is typically closer to what one wishes, as replacements are not necessarily in the same column, but rather determined by pattern.