1988 Ford Bronco Problem selecting gears on1988 Eddie Bauer

Transmission problem
1988 Ford Bronco V6 Four Wheel Drive Automatic 273000 miles

Hi there,
I bought an auto 1988 EB bronco in vancouver and have driven it to banff with little/no problems. It was slipping out of gear when under heavy load (steep mountain roads) but it would down shift when I gave it gas and was no problem.
I checked the transmission fluid and topped it up and drove for another month before getting stuck 2 days ago. The gear slipping never went away completely but now it has come back really badly (as if the clutch was gone but its an auto) the car would move but only when the engine was reving at 4000rpm or so. I checked the fluid again and overfilled by accident but managed to limp it home. I have now drained the fluid back to the correct level and when the car first starts it will select gears fine but when I try to drive away it will act as if in neutral as soon as there is any load on the drive train (i.E. Slight hill etc) it wont even make it out the driveway.
when the engine is warm the gears wont select at all (as if in neutral) and there is an intermittant wizzing noise from the transmission when in gear or in park.
Also when I try to select a gear the engine gains around 500rpm from idle and sits there without engaging the gear.
Any ideas of what is going on? Is it siomething I can fix or is it off to the shop and waht do parts cost?

thanks for your help!
Steve