Transmission slips after sitting for nearly a year

[Dmcinfo]

I've been working on my Blazer for nearly a year so it's been sitting and not driven. I finally got it mostly back together and fired it up last weekend and decided to drive it around the block. I checked the transmission fluid level before leaving the garage and it was very low (selector shaft seal went bad when sitting). I filled it up but the transmission slipped a lot when I took my drive. The transmission shifted and drove fine before I started my restoration. Anyone have any suggestions on what to check or any explanation on why its slipping?

Thanks for any help.
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