HELP needed with GMC transmission problem

My 2000 GMC Sierra truck started having a problem shifting from first to second gear. The other gears are fine. The whole truck shakes when it goes into 2nd gear.
UPDATE: I found out the next day that it was working okay and it hasn't done it again since. So I'm thinking that once the truck is hot, is when it's going to do it again.
any ideas?
Terry in Texas
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