93 4WD surge. T-Case prob??

[tuco]

I recently obtained a 93 Aerostar 4WD XLT, 4.0 V-6, W/ 174k. mi..All going well for the first week until I drove the van after it has had less than an hour or more to cool down after being thouroughly warmed up. Then came a pulstaing, or surging sensation at various speeds, but not consistent with wheel speed. Did in neuteral as well. Auto trans. if I did'nt mention. When the pulse happened on a dirt or gravel surface at low speed, you could hear the rear wheels subtly scuffing or dragging. 1 - 3 mph. Checked the T-case out and the whole thing was wet. In and output shaft seals are okay. It was leaking around the seam where the case halves meet. loose bolts? Anyways, I drained the case and out came about a quarter of the fluid nessisarry to meet the required 4.5 pts.. Fluid did'nt look or smell burned and was a nice bright red. Refilled with the recomended amount and drove. Nervously. But despite all of my finger-crossing the prob is still there.
What the heck do I do? I was wondering if maybe a speed sensor on either axle existed, and if so was perhaps defective. Or the T-case is dead. What type of 4WD is the T-case? Viscous coupling, servo type, etc.. It just fells like a front or rear end will grenade if I drive it. It does not coast very well in neuteral. Or drive for that matter.
A hole rusted the rear brake line through, I replaced the line, bled the system and the rear ABS light is still on. Perhaps I have not reset something or bled it correctly. But from what I understand, if the ABS light is on on your dash, the ABS system will not activate and you brakes function normally. But without ABS. So I have pretty much ruled the brakes out of the equasion.
This is the newest and only american vehicle I have ever owned. Old European are my thing. But I need a van for work and I instantly fell in love with the aerostar. So any help would be greatly appreciated.