a 3hp22 from a 733, 135,000 miles. You can check with one of the tranny guys (Joe G or Kirt K) but I think one of the rear clutch packs is responible for reverse. But it may also be the valve body. If you still have your old tranny, you could try swapping that over???
I'll bet the tranny guys would be able to tell you the cause due to the fact that you had to wait a second when shifting into reverse.