Hey Murphy,
I just did my 241 and I had the clunk in reverse as well (transfer case would pop into neutral). Pretty sure it was the busted factory output shaft snap ring that was allowing the whole shaft to slide about 1/4" back and forth inside the transfer case.
From what I hear, your 243 is very similar to my 241. It was relatively easy to tear my case apart, find everything that was broken/damaged, order new parts from one of many distributors and get 'er done on a Saturday afternoon with a 12 pack.
Get the upgraded output shaft snap ring. Much beefier. Not easy to put on with cheap China-harbor Freight snap ring pliers. (You could also do the slip yoke eliminator.. I opted not to.)
My rig is running smooth now. 4WD is quick and easy.