F150 -- problem with 4X4

Hello. I was transporting firewood yesterday with the 2003 F150 and at one point had to put it in 4X4 low to get up an incline. Transmission is an automatic with automatic 4X4 -- a switch on the
dash -- and self-locking hubs.
I use 4X4 high every winter, but rarely use 4X4 low. I've had the oil changed in the front transfer case according to Jiffy Lube's recommendations.
It worked fine, and I finished the job. But then it would not switch back to 2 wheel drive. I put the switch in 2 but the 4X4 light remained on and it was obviously still in low.
I tried things like stopping and starting, putting it in park and neutral and back in drive, in 4X4 high and back into 2WD, nothing.
So I drove up and down a side-street to see if it would go back to 2WD by itself. Eventually it did, with a huge CLUNK that shook the truck. Now it drives fine in 2WD, but will not go back into 4WD, either high or low.
I guess it's time for a trip to the transmission specialist. But before I go, anyone have any ideas what might have happened, and how bad it could be? As it drives fine and doesn't make any new noises, and I'm a little pressed for cash right now, am thinking of driving it the way it is until Fall when I'll have more money. How bad of an idea is this?
    Ron - http://www.christianfamilywebsite.com http://www.ronaldchristian.com Definition: Nelp: Contraction of "no help". Colloquial: Help messages that are of no help whatsoever. Pertains to help files, messages or documentation that convey no useful information, or pedantically repeat the blindingly obvious.
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first off, never, ever, ever go to jiffy lube. second. there is no such thing as a "front transfer case" third, the transfer case shift motor is most likely bad.this is a common occurrence with electric shift transfer cases, and will not hurt anything to leave as is. the only thing is you will not have 4 wheel drive till it is repaired.

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