Almost certainly not ABS. I don't believe you have independent rear suspension, so not a CV joint either.

If it's acting up while coasting in neutral and not touching the brakes then most likely it's driveshaft related. If you sense it's coming more from the front then either the forward universal joint or the driveshaft centre support (carrier) bearing would be my initial suspects. Most likely it's the U-joint as you didn't mention thumping, which is a classic symptom often associated with the centre support

The symptoms you describe all match vibration and noise issues when the forward driveshaft is having problems. It's not uncommon to have both the U-joint and the support bearing gone so you need to check both. If you replace the U-joint consider doing the centre support even if you're not sure it's gone as it's likely had to put up with increasing vibration in recent years. They're not that expensive and you're working in the same area. Mark all your driveshaft connection alignments before disassembly.

Also check the transmission mount. Shove the driveshaft up and down at the front so see if the transmission can clunk on the crossmember. If the transmission mount is failing then as the trans drops down closer to the cross frame it makes a slight change to the angle of the forward drive shaft and can make a failing U-joint or worn centre support bearing sound noticeably worse during braking and heavy acceleration. If you can't remember the last time the transmission mount was replaced then it wouldn't be the worst idea to replace it at the same time as you do the U-joint and centre support.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now