If the new vent is directly in line with the ring gear, or even close to it, it's entirely possible at high speeds the fling off the carrier/gear is getting thrown and pushed up and out your vent tube.
my suggestion is to pull the cap of the OE vent, cap your ABS cover, and run the vent line in it's original place.
'91 Frankenheep YJ. 4.0, 4" BDS, Richmond 8.8, Titan 12k, 35" KM2's, rust.
'88 YJ, 4.2/4.0 head, +4½" & shackles, BFG 33's (sold)