96 Rodeo won't go into 4x4 Please help

My elderly father owns a 96 Rodeo 4x4 and since he doesn't drive it much or go thru mud trails, he almost never has it in 4x4 mode. Except now there's
a nice amount of snow, and the truck refuses to go into 4x4 when the shifter is put into 4wd high (or low for that matter). The shifter goes in easy, and it feels like it always did, but the light for 4wd doesn't go on and the front wheels definitely don't pull. I'm pretty good with cars but I've never had a 4wd truck to work on so I'm a little stumped. He hasn't had in in 4wd for probably about a year or more. Any ideas what to check?
Thanks
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I have had problems with 4WD on a Trooper, I wrote it up http://www.lssolutions.demon.co.uk/Car/4wd.htm
I also had problems with a poor earth on the solenoids on the front axle.
David

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David wrote:

replace the selinoids on the front axle easy fix for about 60 buck at your local Isuzu dealer took 15 min to replace the old one was all rusted out...
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Thanks Dave, I'll give that a shot. That makes sense why it won't go into 4wd.

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