90 Regal Limited 4T60E transaxel
 

My Regal has a jerk when shifting automatically from 3rd to overdrive, sometime it shifts back and forth between the 2 gears before staying in overdrive depending on gas pedal pressure.
I have been driving with the gearshift in 3rd today and it seems to do OK.
Two transmission shops claim the 4T60E needs to be removed and rebuilt with cost from $1900 to $2300, the other $1600 to $1800. We can't afford to spend that much now!
We bought the car new for my wife and it now has less than 100,000 miles, estimated as the odometer quit at 44,000 about 10 years ago.
My wife loves her Regal and we can't afford another one.
Since, to me, that this is an overdrive issue can I extend the life by leaving it in 3rd gear position and take it easy with acceleration?