this is kinda funny the low beams dont work but the high beams work. i replaced the bulbs and check on the fues then did some reseach came to found that someone had the same problem but didnt fix it.
Would this be a expensive thing to get fixed? should i just drive with high beams on and leave it be?"thats what the last owner did i bet" really kinda funny when you buy a truck during the day the headlight where the last thing i was thinking about. It doesnt seem to be a huge deal but I should consider fixing it. I always wonder why i see some people driving with there high beams in town. also parking lights and brakes lights work

 

i had the same issue, i tracked it down to my pull out headlght switch on the left corner of the dash... wierd issue too- i pulled the switch and when i did, i noticed the block was melted a little, i went to the local junk yardand checked another one out and it had the same issue. I replaced the headlight switch with a new one... same problem, then i took it out again and played with the wires with my switch pulledout and they blinked a few times till i held my tongue right as u could say and they stayed on. put it back together and problem solved.

im pretty sure since your truck is a 98' it might be a little different but same concept.....maybe same issue?

 

ok this is wreid but it fixed it self. heres what happen i was out driving this mourning with my high beams on and i stoped for a bite to eat and went back to my truck started the high beams work but wont stay on and now the LOW BEAMS work great! maybe some magic cummins certified truck gnome fixed lol

 

+1 on the pull out switch.I have put two on my truck. China replacement part didn't last to long. Mine also fixed itself off and on.It was getting hot and wouldmalfunction.

 

Every so often my lows go out as well. I just flick the high beam switch a couple times and they come on. I believe the switch itself becomes gunked up.