rebuilt transmission or put in another one from junkyard?

Looks like the transmission just went on my Oldsmobile Ciera (1988) 2.8L MPFI engine. I am thinking of driving this car for another year or so.
Just today when I had to stop at a red light on an uphill. I was
horrified to find out that as I lift the foot off the brake pad, the car started rolling backward. I quickly stomped on the brake (lucky for the guy behind). So that's that, its not working like it should and I need to service it asap.
I understand if I take it to the shop, I have the option of repairing the transmission or take one (with low miles) from a junk yard and put it in instead.
Can you please give me the pro and cons of the various options? If I take another one from the junk yard would it fit properly between itself and the engine? Or maybe I am better off just rebuilding the transmission in the car.
PS: I had the transmission filter and fluid replaced in the car 3 months or so ago. As well as replacing the engine and front & rear transmission mount at that time.
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Stephen wrote:

<snip>
I'm confused... if I stop on a steep incline, like for instance on the ramp from an underground parking lot, mine will (would) surely roll backwards were I not to keep on the brake... I think every car I've ever had does.
Is this not normal?
Ken
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Well I use the underground parking lot everyday, and I know for a fact it never did this before. This car is automatic 3 speed and it was in DRIVE when this happens. Normally for this incline it never happened before.
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If you are on a hill, and in gear, and take your foot off the brake, the car SHOULD roll backwards. Because the engine RPM is not high enough to keep the car from rolling. If it didn't roll on a hill I'd be concerned. Your engine, at an idle, in drive, should not have enough power to hold the cars weight on a incline. None of my cars have .. maybe something was wrong before?

DRIVE
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This really varies widely by model, hill, trans, engine, etc. But, if a particular car didn't used to do it but does it now, either the fluid is low, or the tranny's going (slowly) GW
clevere wrote:

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Or the idle speed has changed

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David J and Lynne J Shepherd wrote:

Hi...
Went shopping this morning at superstore. Exit from the underground parking is quite steep; there was virtually no one else around, so I experimented.
Stopped on the (about 20 degree) ramp, and applied gas to hold it still. Took lots, felt uncomfortable. Part of that may be because I was in a reverberating concrete tunnel so to speak, but it was lots of rpm's.
Take care.
Ken
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