Transfercase problem...

My bud and I just went and looked at a 1990 XJ with 4.0, automatic, 4WD.

It went into 4HI just fine, but while trying to put it into 4LO it grinds like a SOB, and shakes the jeep alittle....

Any ideas what is wrong? Also, they said it used to be towed behind a motorhome, so that might be a contributing factor...

Was the trans in nuetral when you tried shifting the case?

yes it was

I have the same here, and it sat in a garage/driveway for about 3 years until my borther gave it to me. Mine did that in both 4H and 4L. I think usage is a factor. It no longer does that as I used both on and off all winter. Another factor, is if youre wheels are slightly turned, they can do that, or if you're going too fast.
It may be one or a combinatoin of these factors, and not necessarily something is wrong. I'd rule them out first as they're pretty easy to do. I was worried something was wrong with mine, but turns out everything is fine, and now with use, changes easily between each.