the drive shaft on my 1975 chevy suburban catches and/or bucks when i put it in gear or when the driveshaft is turning

inspect the ujoints...these have tiny little needle bearings that can lose their way...sometimes~ https://www.google.com/#q=1975+chevy+suburban+U-joints&tbm=shop

the u joints look new. last owner says they were replaced. what exactly should i b looking at and do i have to dis assemble the joints to properly inspect them?

place your vehicle in park chock the front wheels. jack up the back end so both wheels are off the ground and leave vehicle in park.. turn the driveshaft by hand ( there wont be much turning room) try and move the driveshaft up and down by the rear differential and turn quickly back and fourth and check for excess amounts of play. and do the same with the front.

....well let's hope ta gods it's not your differential...troubles with ALL my '66 Chevy truck differentials where the ring gear was not engaging the pinion...and there needed to be special shims to put it right...shame you couldn't run it with the two wheels off the ground...get under it..to see what's the deal there~

could it be trouble with the the slip yoke/ grease fitting/

This ain't NO 4x4, is it?

You'd have to inspect with ye own two eyeballs~

if you've got zerk fittings and are an old timer like me armed with a grease gun could squirt fresh until it comes out fresh~

yep its a 4x4

okay initially you could take it to a shop with a large single hydraulic piston and a couple of pads on the ends of a swiveling x-brace (like the kind I used to work with) where you can have all four wheels danglin' and push and pull and scare your friends as you discover whatever LOOOSE components exist and need your attention~

thanx alot. appreciate the help. ill try those things. hopefully its something simple

idle is too high should be about 800 rpm or. positrac additive needed in rear end, or warped ring gear in rear, throw neutral, jack it up and turn it by hand see it happen check rear torsion bars also