An "8 pinion differential" refers to the number of pinion gears in the differential.
Since about the early '70's almost all Wheel Horses have used either a 4 pinion or 8 pinion differential. I'm thinking there are a few models of 1 1/8" axle transmissions that used a 4 pinion diff, but most used the 8 pinion.
The 8 pinion is regarded as the most heavy duty rear end available, but the 4 pinion diffs are still pretty stout, and are more heavy duty than the spider gear diff that you have just blown apart, or the 10 pinion limited slip diff that was around in the the late 60's.