You did not give the model numer but I think older Simplicitys did not have wheel differential - only pinned hubs.

If both pins are going through their hubs and the axle then the pulling may be due to tire pressure.
It can be a little presssure for grip, a lot for ease of handling but even for straight running.

David
http://www.kedawei.com/snowblower.htm

Jacks look up has a good parts diagram for your machine. It shows as a straight axle drive. If both wheels are pinned through the axle and pressure up they will be driving. Assuming your snow thrower doesn't have the decal that says 9/28 RT that only turns right (heh heh ;) Then I would look at skid/ scraper adjustment. The right hand corner maybe digging in. Hopefully not, but possibly the whole machine got tweaked along he way.
In regards to the traction beginning in 3rd. It is likely either an position/ tension adjustment or that a support bearing is worn. Maybe the friction wheel is worn as well. Look at the driven disc and where the friction wheel is in relation. The disc should be flat and not wobbly. The bearings in the support shaft should be firm. This should get you started. Good luck.

Here is a link that might be useful: Jacks

I pressed my old Ariens into service the other day. I noticed that it was pulling. I'm pretty sure one of the wheels needs air, and I'll check out the skids too. It goes forward great , but is slow backing up. Does this sound like an adjustment. TIA

I would think so. It maybe helpful to drop the cover so you can see if it travels over far enough.

My first suspicion was tire pressure so I filled them both to 20# (per manual). My second was the scrapper but it does the same thing when I lift it and the scrapper isn't touching.. I have noticed it seems to have a little surge at times. I suspect an adjustment of some kind OR maybe the bolt going through the axle (per drawing) has sheard and is only driving part time.

Thanks for the responses. I'm sure I'll get it figured out sooner or later. My biggest problem is I'm doing the work in an unheated shed...too many trips into the house to stand by the stove.