I've been searching the net for the past week and a half now for somebody with the same issue as I now have, but I've had no luck.

Here's the situation:
I let somebody use my truck for hauling some furniture one day, and when I got it back I came across a transmission issue that was not there before. ( not that it had any issues before. )

I have the three speed automatic with overdrive in my 91 chev pickup. I generally use the tranny to slow the truck down when driving due to crap brake pads and the awesome sound of straight pipes. However, after I got my truck back I noticed that when I dropped it down a gear... nothing would happen... the truck would just coast until I applied throttle.
This goes for every gear, shifting from OD to D, D to 2nd, and 2nd to 1st. The truck Will NOT *manually* shift down.

Now it would seem normal to have messed something up that it would not shift into a gear, but that doesn't seem to be the case. When I am in drive, the truck will shift up and down perfectly when and where it needs to, and I can hold the truck in a specific gear by shifting *up* to it manually, if I so choose. I just cannot shift down to slow the truck down.
It goes into gear by itself at the correct RPM every time and it shifts smooth.

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So besides the down shifting issue, here is whats confusing me the most.

Before - I could put the truck in first and speed up to 40, ( or shift down from a higher gear ) then let off the gas and the use the tranny and the engine to slow the truck down again. The most noticeable thing would be the nice sound the truck made, and that I could slow down in a tenth the distance than I could with just my crappy brakes.

Now - I can put the truck in first gear and speed up to 40, but when I let off the gas... the truck just coasts. The truck is Still In First gear and the engine should thus work as a brake when i let off... but there is no resistance at all. If I apply throttle, it speeds up, but dropping the engine RPM no longer slows the vehicle at all... This is confusing the **** out of me because I have not seen this happen before and cannot understand how that's even possible, even though it's probably something stupidly obvious.

As far as I was aware, the transmission is a direct input-output setup, with the addition of the torque converter. So if you spin one end then the other end will spin, regardless of which end you turn. It seems to me that it's behaving in such a way that the input drives the output fine, but when you spin the output, the input does nothing.

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I've done extensive work to many vehicles, but automatic transmissions are not something I specialize in... obviously.
If anybody could give me some pointers, or point me to a thread that I've obviously missed, it would be greatly appreciated.

Thanks in advance.