Best bet is the slip yoke on the drive shaft. If your U joints are fine, and your pinion nut is torqued down, and your drive shaft flange bolts to the transfercase haven't backed out, then it's the slip yoke.
Pull your drive shaft down, get a big hand full of white lythium grease, smather up the splines, stick it back together, bolt her up. Make sure you put the boot back on.
2000 Ford Ranger XLT: SAS, Sliders, Bumpers, Gears, Lockers, Shafts, 33gal gas tank, Shortened from 130 to 112" WB, Home brew bed and spare carriers,
Receiving: tons and a 351w stroked to a 427.
1972 Ford F250 "Explorer", 390fe Big block, lime green
Receving: 14bolt rear w/ disks, cam, 650cfm 4 barrel, intake, exhaust,