In High you can go 55 just fine. Id recommend slower if your on slick roads obviously. In Low you'll top out at about 35.
Also if you have the part time or full time option. Go with full time on the Highway. It will handle higher speed better.
2000 XJ Sport Freedom Edition (whatever that has), 32x11.50 Bighorns, 4.5 RE lift, ARB Bull Bumper, 2 KC slim's, M8000, custom safari rack, Detours Tailbone, GOLIGHT