Welcome! Get it on some gravel and give enough power to spin the tires. If both spin your good to go. DL's are fairly bulletproof. But they can be scary, having had one for 15 years. Sometimes a sudden acceleration or let-off on the go pedal while on pavement will give you a quick sideways jerk as there is lots of slop in the differential occasionally causing one tire to do all the pushing or pulling. Mine chirps the tires on every acceleration while turning and pulling out into traffic. And on a slow turn in a parking lot you will hear a gravelly noise like a bad bearing. And the worst is every once in a while, the tires will bind up on pavement and when it lets loose, it will sound like you broke something. Not real user friendly, but good in the sand.
'84 CJ7, '07 5.3 LMG Chevy, D300, 4L60e, 33's, 4.10, rear Detroit Locker, FG Tub, HOK Tangelo Pearl