1999 Explorer Transmission

My wife was driving our '99 Explorer, 4wd SUV to town the other day. 
When she got back she said the OD light had come on. I asked if she had 
inadvertently pushed the button in, she said no. After shutting off the 
vehicle and restarting, the light went out.

I took it for a short drive and noticed also that when shifting, it 
wants to rev a bit before shifting, then instead of a smooth transition, 
it shifts hard into the next gear. Never did this before, always shifted 
smoothly. The vehicle has 130,000 km's. When I got back to the house, 
the OD light was back on again.

Any ideas what may be causing this?

Thanks

Allan