If it's 4wd you got a driveshaft that goes from transfer case to a flange at front differential. There's a ujoint that connects the driveshaft to the flange yoke.
How can you check it? I would simply unbolt those 4 bolts and remove that driveshaft. You don't need it unless you are using 4wd. Drive it and see if the problem is gone. If it is then it was that u-joint. They sell replacements that have zerk fittings. That way you can regrease ever so often and supposedly last longer. Good luck
The reason that's my 1st guess is that you feel it right under you in floor board and because, like me , it got better when accelerating.