I had a look at the link the OP supplied. There doesn't seem to be any description of what they mean by the phrase "knob style differential". From the discussion on this thread I get the feeling that the unit is a axle mounted pin lock system that has a knob on it. If so then this is NOT a DIFFERENTIAL.
A differential is more than just releasing a wheel so that it can free wheel. Its basic purpose is to allow two or more wheels to go around a turn and have power and speed properly allocated to the wheels. The wheel on the inside of the turn turns at a lower RPM than the wheel on the outside of the turn. Its mechanically more complicated and much more sophisticated than letting a wheel run free. A limited slip differential alleviates the problem that occurs when one of the wheels start to slip on ice or in mud. With a normal differential the slipping wheel gets all the power and you are stuck. With a limited slip differential all the driven wheels are guaranteed to get some of the power and hence you are less likely to get stuck. This is normally in the context of vehicles, the only snow blowers that I know of that currently have differentials are ones manufactured by Ariens. If I remember correctly John Deere did hire Ariens to manufacture snow blowers at one point and I think some ( or all, I'm not sure) had differrentials. I'm wondering about the current crop of John Deere units.
Can anyone confirm whether or not the John Deer 928E has a true differential?
The 928 does not have the easy turn feature. I was not able to contact John Deere to get a good explaination on the "true differential" question: They did return my call but I wasn't available when they did and I haven't called them back yet. Here is a pdf file that shows a nice picture of the 1130's Easy turn feature.
http://www.deere.com/en_US/ProductCatalog/HO/media/pdfs/jd_snowthrowers_brochure_eng.pdf
The 1130 is the only one in their line that offers the Easy turn and it is again not on the 928E: Here is the JD website where it gives specs on both units: http://www.deere.com/en_US/ProductCatalog/HO/servlet/com.deere.u90785.cce.productcatalog.view.servlets.ProdCatProduct?pNbr=LP1695442&tM=HO
