I have a 2002 Supercab M5OD with 125K miles. Have needed lately to frequently engage 4HI and have had problems. Lever moves crisply into 4HI from 2HI and I can hear a "thunk" but 4X4 light on dash doesn't come on. It turns out if you park and shut off for a while, on startup 4X4 light is on and 4X4 is engaged. Almost the same in the opposite direction....if you are running in 4X4 and wish to go back to 2HI, after shifting the lever back to 2HI the 4X4 light is still on. It feels like 2WD but the light is still on. Let it sit a while, and on restart the 4X4 light is out.
I love the truck (it's my third) and enjoy this forum, especially posts from JMC.
Thanks

 

man the first thing i would check is my vacume lines.
I would shift mine and hear the clunk but still be in two wheel drive. Its just one of problems that i have delt with. First take out your battery and battery box and trace your vacume lines. thats how i found my problem. But yours may be different. but i would give it a shot.

 

Thanks for post lewdog. I'll give that a shot. Been so cold here and snow every 3-4 days, too cold to work on truck outside right now. Maybe a vac line is kinked or leaking. I was thinking the switch is gummed up somehow and slow to respond. Been using the 4X4 with all the snow...nothing as embarassing as driving a 4WD pickup in the snow in 2WD with the rears spinning. Thanks

 

it does sound like a possible vacuum issue or possibly something wrong with the shift fork in the front differential. I would assume that your transfer cases is engaging and dis-engaging, but the front diff isn't locking the half-shafts.

Good luck and post your results!