Craftsman LT4000 - Only one blade engages

I have a Craftsman LT4000, 42" cut. When cutting my lawn for the first time this season, I noticed that only the right blade spins and cuts. I checked the belt, and it appears normal. Once stopped, off, and disengaged, I noticed that if I move the right blade (the one that is working), there is some tension and it only spins as far as I move it. If I spin the left blade (the one that is not engaged or cutting), it spins freely if I push it. Any suggestions with what may be wrong?

Follow-up: I found that the spindle is broken (I think). How do I swap this piece out?

Remove the nut, and replace the pulley and most likely the shaft because it will be damaged from the pulley spinning on the shaft.

Hold the blade with a gloved hand or block the bade with a lump of lumber the use a long wrench or impact on the nut.
The shaft comes out from the blade end and you might need to dress the end of the shaft if it has burrs on it that stop it passing through the bearing

As per previous you will need both the pulley & the shaft at a minimum.
While it is out pop new bearings in as well.

Go to a mower shop with your old pulley & spindle as there are a lot of different diameter pulleys and it is important that both blade turn at the same speed.
Do not buy a cheap one off the internet.
There are a lot of defective shafts and complete spindle assemblies out there, most likely the ones that the QC in China rejected then the whole reject batch gets bought as scrap then filtered back into the retail market.
This applies 10 fold to spindle bearings.
I had a customer who bought a "full set" of spindle bearings off evilpay the trashed his deck when they failed because they were shopping trolley grade bearings .