I've searched the forum and not really found what I'm looking for, maybe somebody can point me to a related thread.

my new to me 98 jeep xj sport, 4.0L, AW4, stock height, 225/75R15(fairly new), 163k Mi on original engine trans etc.

I found the truck makes a cycling droning noise between 60mph and 70mph(100 - 120km/hr), it's not overly loud but noticeable, there is a slight vibration throughout the truck that is more continuous than the cycling droning noise. I'd say tire balance for anything around this speed but the tire balance does not "cycle".
the cycle is very consistent, kind of a wraaoooow wraaoooow that is one second on, one second off, repeat. haven't pushed it too much past 120km/hr but it's cycle does increase relative to speed, I notice nothing under 100km/hr

any thoughts?
my vote goes mostly to the rear gear set, since it is the only thing that I can think of that has a cycling variable(small gear turning a big gear) but transfer case also qualifies under that definition. my feeling thus far is that the issue is coming from the rear, but I've had other vehicles trick me with a sound/feeling that travels through the vehicle leading you astray

 

harmonics from tread slap and body vibration...?

 

anyone? I know this is not an uncommon noise, just want some input

 

Id check front and rear drive shafts for play.

 

Sounds to me like it could be the tires. If there's any difference in tread the acoustics of the road noise will do that. When the sound waves are slightly different wavelengths they will amplify and cancel each other creating a sinusoidal fluctuation in the sound.

What are you running for tires? If you have a set you can use or borrow swap the wheels and run it and see if it makes a difference.

 

You may want to read through this data. It's from a Dodge truck site but I think this axle noise diagnosis data could pertain to any vehicle.

http://dodgeram.org/tech/repair/axle...sis/page1.html

 

got a chance to take her out on the highway again, it's more of a rumbling, it does definitely cycle and I feel it's more coming from the front than anywhere else.
just picked up winter tires on rims, going to get them balanced and toss them on and see if it goes away.
since it is cycling I have my doubts on it being tires but it's time to throw on the winters anyways, so why not rule it out.
Id it's not the tires then I imagine it could be front drive shaft, so will try removing it and see what happens.
I've heard before that if the front pinion angle is off it can result in rumbling and vibrations, often in a cycling form.