95 Jimmy tranny prob/service engine soon light

Can anyone help me. I have two problems with my 95 jimmy 4wd. First is the transmission. It shifts really hard when you shift into reverse, just reverse. Any thoughts...Also the service engine soon light came on and i scanned it and it said o2 sensor bank 1 sensor 2. I figured it was the sensor right before the cat. That's also what gm said so i changed that sensor and reset the computer and it still comes back on. And at $75 a piece it isnt cheap to just guess. Can anyone help...please. Thanks a bunch.