need help

So ii recently bought a 99 with the inline 6 with 187k miles I payed $1500 for her she came with a six inch lift and 33"PRO COMP TIRES NOW WHEN I FIRST BAUGHT THE JEEP 4-HIGH AND 4-low worked just fine after taking her mudding a few times no I can only get 4-high when I put it in four low it s into 4 high so no matter what I put it in only four high engages and to be honest I am just learning to work on vehicles any suggestion will help please and think you