Intermittant "thunk-y" noise in reverse
 

My '94 Wrangler (4 cyl, 150,000 miles, manual trans.) has started
making a wierd noise when I am backing out of parking spaces.
Something is hanging up inside the drivetrain, and once in a while, it
is bad enough that it almost stalls the engine.

Today, I jacked it up and tried turning the back wheel by hand. At
random intervals (about once every ten turns or so) it gets almost
impossible to turn for a moment. The noise seems to be coming from the
transfer case, but it might be coming from the transmission, or maybe
even the differential.

FWIW, The car runs perfectly smooth in all the forward gears -- except
5th, which stopped working about a year ago.

Suggestions, anyone?? TIA, john w.