Here are some links that may help you out the first one has the ATSG manual listed for download and the second has the detent repair kit listed part way done the page.
http://www.mymopar.com/index.php?pid=22
http://www.transmissioncenter.net/727transmission.htm

Good Luck

Here is what the input shaft looks like with the ball in the center.
The ball is there there to block an oil passage if I have understood it correctly. The thrust washer between the input and output shaft is supposed to carry the load, again just my understanding.

The flat spot in the ball was caused by the machine shop going a bit too deep when replacing the bushing.

This here's better pictures (haven't had time this week) Notice the ball is pretty mangled , not even close to round , still looks like a mangled bearing, dunno. Is it too small for the detent ball ?

Nuther shot

obviously it's been banging around in there loose for a long time & the trans was working OK (for the most part I'd think) for quite awhile for it to have been up & running long enough to do that amount of long term damage. As oldmanmopar said if a caged ball bearing cage lost a ball it've shredded. To me it sounds like either a ball bearing got loose that wouldn't affect the trans operation & I'd think all of em would need to be accounted for for proper operation or an extra ball from another trans at a tranny shop got dropped inside during the overhaul but that dont seem likely . That mystery may not be solveable. Myself I'd open it up including the valve body & verify that every ball is there that is supposed to be there & if so button it up & call it good.

10-4 , better safe than sorry , probably be good experience anyway as I've never pulled a valve body before , just want to get this thing back in the car ! Always something .................

only caveat on the VB is you want it either right side up or upside down (I forget which) & is that referenced to with the tranny belly up horizontle on the bench or as it'd be in the car, so when you seperate the 2 halves the balls that are in there dont fall out but instead stay in their pockets to be easily accounted for

Valve body removal is fairly easy first you should go here and download the A727-A804 torqueflite service manual, it gives in detail all about repairing these transmissions.
http://www.mymopar.com/index.php?pid=22

If the detent ball is missing you will see that without taking the valve body apart.

If the spring section is in good condition you can order just the ball and possibly the spring to get it back in good working order.

This link is to Jegs for the TCI 220000 shift kit if you click on the instruction s you can download a copy that shows in general how to disassemble the transmission while in the car.
http://www.jegs.com/i/TCI/890/220000/10002/-1

Take your time and don't force any parts and you should be ok with this repair.
Good Luck