Have a 2001 with about 115,000 miles. For about 2 weeks the transmission would grind and rattle when I engaged 4 wheel drive.

Yesterday when I engaged 4 wheel there was a large clunk, and now switching to 4 hi or 4 low has no effect. The front wheels don't engage and there is no noise.

Any ideas about what happened or what repair costs I can expect?

Thanks!

Sounds like a problem in the transfer case itself. Putting your truck into 4wd should not have a huge effect on the trans except for maybe some bearing whine, or planetary noise. My guess is that something in the t-case bound and broke. There is a monsterous chain in there that I've seen break. Basically the chain connects the input shaft of the t-case (which is driven by the output shaft of the trans) to the ouput shaft of the t-case. Since there is no movement, it's possible that the chain went. They are usually very loud (rattling/grinding/clunk) when they go. Or you could've broken some gears or a shift fork. It sounds internal to me. Good luck...

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2002 Dakota Sport, 4.7, headers, k&n, 4.10's