Rob, I can attest from personal experience on my 2001 or 2002 rubicon (not sure of year) that they are very very solid ATV's!
To answer your question there is NO difference in them from their initial release up until this last 04 release (04 supposedly had softer suspension). For 2005 they added traxloc (2wd/4wd) and disk breaks along with a "T" shapped seat for added comfort.
As for my story, when I purchased my grizzly I had gone to get the rubicon and had even called ahead and had them holding one for me (2004 model). Once I got there the only reason I decided on the grizzly was IRS, locker, and at the time 2wd/4wd switch, where the rubicon had none of these.
During my owning of the rubicon I rode it harder than any ATV I had ever had!! I had huge 27" ITP 589's and rode double everywhere. I was the idiot that when there was a mud hole I was the FIRST one into it wide open and ran WOT most of the time while shifting in the upper RPM's and sliding the thing in donuts ect. with those huge tires. I also used it to "make new trails" by running over 2-4 inch in diameter trees (this did crack the plastic on the front bash plate). I never had a single problem in the 2000+ miles I put on it. All I did was change the oil and that was it. The only reason I sold it was at the time I needed money for my wife to get a new car. The transmition is probably more soild than ANY on the market.
Does any of this make me want the rubicon over the grizzly....Absolutly NOT, because for the type of riding I do the grizzly is best for me. however, If i was to want a utility ATV for plowing, hauling, or general hardcore work (with strain on the drivetrain 24-7) then the rubicon has the BEST SRA with Hydrolic transmition on the market!
I hope this has helped Rob, but have you looked at a Rhino??? That is my next purchase and it is perfect for plowing as it weighs 1100lbs and has a dump bed for all your gear! It also has the exact same setup as the grizzly. However, STAY AWAY from the 2004 models as they were buggy from what I have read on many forums but for 2005 they fixed all the bugs (ie. air intake from above engine not below, and larger axles and added glove box door along with the wires for the tailgate support like trucks)
again here are some photos of my old rubicon right before I traded it for my wifes car..