I swapped all 4. They were fairly inexpensive at the "jungle website". I ordered name brand BOSCH parts. $212 all in - for all four including tax and delivery. Delivery took a week though.
Job took roughly 2 hours all in. Including rearranging cars and such. Hardest part was getting my hands on the sensors to depress the clip that holds them in. Passenger side rear was by far the worst.
There is a difference in length at least on my 2000MY 986. The rear is longer and black Part # 15183. The fronts are blue Part # 15182
The car has 96480 miles on it. It was throwing O2 codes for years. But they were truly intermittent. As in years would go by without a code. Even this year, the codes would come and go with me doing nothing.
But I will say that the car is noticeably sharper.
Hard to explain exactly, but the engine is just better. On an on-ramp, there is not the slightest hesitation. As there was before - and I mean slight - most people probably would not notice. But driving back to back, you notice.
Bottom line, definitely a straightforward job. The Harbor Freight 22mm O2 socket worked just fine. The jobe would be easier if I had a lift and more flexible wrists. If you have a choice, jack the car higher than you would for an oil change. I was on ramps, but If I had it to do over, I would want to be able to roll around better to get my arm into tight spaces.