Jack Up and Support your Vehicle on Safe Jack Stands. Then after laying down some Cardboard to make things easier, slide under there and with a decent Bright Flashlight (Yup... even in Ambient Daylight) ...examine the "U" Joint Needle Bearing Cup Retention Fasteners for being "Finger Tight" and loose while you push and pull on the shaft enough to reveal any mechanical failure. If those fasteners are Snug, then attempt move the Tail End of the Drive Shaft around to see if the inner "U" Joint Bearing 'Axles' are loose and can rattle around or be gyrated abnormally.
While performing this part... (Wear Eye Protection while under the Vehicle) ...a Dead Giveaway Symptom is any sign of "Orange-Brown Dust" falling out from around or coloring the edges of the Old Needle Bearing Cups. If this is so... Your Un-Lubricated, Old Needle Bearings have been Pulverized by Torque ...into Powder. So plan on the need to completely remove the Drive Shaft, Seal off the Open End of the 4L60E Transmission from leaking Fluid. Then, get ready for a job that might SEEM EZ2 Do... but is one which will require no small amount of manual dexterity and coordination... along with some Mobil1 Synthetic Cup Grease to Make the New Needle Bearings behave and STAY right where they are supposed to be.
If the Yoke End "U" Joint is also Golly-Wogged and damaged in any like fashion, then replace THAT one as well. This tedious and PITA work is almost impossible to accomplish anywhere else other than "On The Bench". If any of these conditions are present, losing a Driveshaft on the Highway becomes a real and dangerous possibility, so if these symptoms are confirmed, the sooner it gets repaired... the better. Other Members with 4X4s can advise you on how to repair the additional Drive Shaft and CV Axles if instead, they turn out to be the Culprits and the symptoms lean more in that direction. There are also a few decent Youtube Videos demonstrating this Repair.
@rdamilkman While you have it off of the ground for MRRSM's suggested steps. I would also check the fluid levels of your Transfer Case, Front and rear Differentials. If you have never changed those fluids I would look into doing changing those fluids AFTER you fix your current issue.
I had weird noises at low speeds (25-30 mph) that ended up being my transfer case making all of those noises. I have AWD though so, that's probably not be your issue. Just good maintenance to check those when you can though.
What fluids did you use in the transfer case and the front and rear diffs? The transfer case basically requires ACDelco Auto Trac II. The diffs you can use ACDelco 75W-90 or another higher end oil like Mobile 1.