I have a 84 gmc truck. Will the leather buckets out of 01 fit in it

sure

yes
you may have to washer it so it will sit right and some times you can change the sliders so it will look better

Yes they can fit. You will need to modify ur seat brackets & install them on the leather buckets. That is if u already have buckets in the 84. If not, go to the wrecking yard & grab the bucket seat brackets($10) from a similar truck. l swapped in bucket seats from a 90 is Baretta into my 82 silverado. l just used a grinder & a handful of screws/bolts & a couple hand tools. It will be obvious when u have it in front of you. You will see what needs to be moved for u to put the seat brackets on the seats. You may have to drill a few holes for the new brackets though. There may be another way, but that is the way l did mine. Good luck.