'89 Ranger trans on the way out?

All,
Since the weather has started to turn cold (aka frigid) here in the midwest, my '89 2.9L V6 Ranger trans has started to act up. Upon start up, letting the truck warm up a bit and then backing down my driveway
in reverse, I shift to drive and the trans seems to be stuck in kind of a neutral state. There I sit helpless in my street for about a minute until something finally catches.
I've experimented starting the vehicle cold while the vehicle is still parked (with the emergency brake enabled) by shifting from reverse to drive and can feel the trans engage in both directions several times. Thinking everything is OK, I shift to reverse, back down my driveway only to be stuck in neutral again in the middle of my street while waiting for the trans to engage.
Only after revving the engine a bit and shifting between reverse and drive does the trans finally engage while I'm still sitting in the middle of the street. Lately I've been backing up my driveway so that I can pull straight out. Only problem is at work the way the parking is designed I have to back up and the neutral problem re-appears at the end of my day.
A little background on the truck. It was bought a few years back with over 100k on the odometer. With that kind of mileage I didn't want to flush the trans fluid seeing how that can introduce other problems. The truck now has over 140k on the clock and runs especially good and strong during the warmer months. I've never had a hint of a trans issue from Spring to Fall and the trans fluid level is fine. Is it time to have a trans flush and take my chances or bite the bullet and let a place like AAMCO tell me I need a new/rebuilt trans?
Any help, especially now in the colder weather, would be GREATLY appreciated!
Greg
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Bottom line: Flush or fix; if the first doesnt work, You take the trans in for an overhaul, right? Or you go straight for the overhaul
Dont flush it, just run all the fluid possible out of it into a bucket by using cooler line.... then change filter, and fill it with new.. plus a bottle of Lucas AT Fix.
What are you out? Maybe thirty five bucks doing it yourself..

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and SURELY NOT AAMCO
try here!
http://www.atra.com/source/amembers/directorysearch.cfm?searchtype=reb
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Try a bottle of Lube-Gard in your transmission, this stuff works wonders! You can find it at Napa.
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