Belt may have a burnt spot on it if the deck plugged once and the engine continued to run against it which could cause the vibration.
Mike

As noted above, belts many times are the source of the vibration. The drive pulley can burn into the sides of the belt , and as the tension is kept on the belt- it gets pulled deeper into the pulley causing the vibration. The 46 isn't necessarily a vibration free deck to start with, but an imperfect blade or belt seems to magnify that vibration.

Now that you mention burning of the belt, I don't really see too much discoloration on the belt, but sometimes when I first engage the blades I see a puff of smoke near front of tractor down low around the muffler and I can smell something like burning. Are there any aftermarket blades or deck drive belts I can purchase with confidence, Sears is high on their deck drive belts?

The question would be if the smoke is coming off the belt or a puff of extra emissions from the engine. Take the Sears part number and do a search. Sometimes a regular AYP/ EHP part can be less. Generally Stens belts in this app is good. Same deal with the blades. Stens blades are made stateside now. OEP (oregon) makes a good blade.
I don't know exactly which model you have , but if your deck belt is a 405143- Stens is a 265-106.

Check the engine drive pulley for separation at the deck pulley location. I have seen them begin to separate and cause a vibration.