Thanks for the tip Conner. I've checked the vacuum at all stages, including WOT (wide open throttle). I get a couple of inches of vacuum at WOT but apparently not enough to open the depression chamber. It looks like I need just a little bit more vacuum but can't get it. I've even tried Teeing into other ported vacuum lines with no luck.

'97 Jeep Cherokee XJ, Country, Black/Gray Leather, 4.0L, 4WD Command-Track, AW-4 Auto, 3.55:1 Gears, PowerTrax No-Slip Locker, Hi Country Tow Brackets, Spectre Air Filter, 235X75R15 Goodyear Wranglers, 126K miles