Very commen with any jeep that sits outside in the snow (exspecaily the vacume engauging ones). If one want to force it into 4x4 hi your best chance is to, pop your truck in netrual (not Moving) and pull your lever in 4x4 low and it should engauge, your truck will want to crawl forward instanly even with breaks full on (love jeeps), now when you push your leaver into 4x4 hi you should be good to go. Works 75% of the time. Other wise your rig just needs some time to warm up or you have a significant vacume leak.
Regards,
D