Transmission Shifting Problems

My 2000 2.5L V6 Auto has about 140K miles and was working perfectly. I had the trans flushed about a year and a half ago but found that the problem was the shifter position sensor. Not a problem since. The other day the trans decided it would no longer shift into O/D. It also is "late" upshifting 1 to 2 to 3 if it's under normal to heavy pressure. If I take off with less throttle and/or back off it will shift normally, i.e. no bang or slip. In final gear with cruise set it is solid with no variation in rpm. As I said this came about abruptly. I do not race the car. I do not lug the car.

I took it to the local Mercury (and many other brands) shop telling them I thought it was probably something electronic and described how it was malfunctioning. I asked for diagnostics. They told me they couldn't identify exactly what was wrong and that I needed a rebuilt installed for $3100. I picked the car up today and paid them $140. The only evidence of any diagnostic they could show me was a piece of the service manual and a little checklist with mechanic check marks on it.

Have I been mistreated? Do these guys just want me to buy a car? Can anyone tell me what these specific symptoms may indicate other than the transmission is trashed.