reverse? puh-lease
. haha... these always hate going into reverse. maybe you got lucky and for years yours has done what you asked of it. i got a marlincrawler rebuilt transmission that shifts like a dream in all but one forward gears, and still fights me getting into reverse. i start to suspect that there's some design issue with the reverse shift arm, like it should have gotten more bolts or better leverage or heck idk.
concerning clunky, i'm guessing you may be experiencing what i call 'notchy' -- it does go into that gear, but it doesn't feel smooth or cushioned. my T4R has that issue to some degree, and replacing the clutch (another marlincrawler heavy-duty kit) did not do anything to solve the issue. i wasn't expecting it would, and it didn't. i have read that 'notchy' is due to wear in the transmission.
if you are wondering what happens with clutch going out, here are the symptoms i've personally experienced with clutch and clutch-related components going out:
pressure plate: sudden lack of all power, yet RPMs stay up, whizzzzzsh sound as the engine revs with no load whatsoever, then after a few panicked moments it's back
pilot and throwout bearings shot: horrid squeak like an oily or loose drive belt only when depressing the clutch pedal, and happens between any gears (for shifting reference). eventually the bearing dries out and starts to smoke as it burns up and fuses to the shaft
slave cylinder puked: depress pedal and there's no resistance, and it goes all the way to the floor and the doomed feeling occurs since murphy's law requires it to happen on the way to work or an interview
worn shift lever cup or shifter bushing cap: pops out of gear under any load, even slight. holding the shift lever in place solves.
hope this helps. i think what you're describing sounds like what i am noticing in my T4R, and unfortunately what fixed mine (same R150F transmission, different vehicle) was the marlincrawler rebuild.
edit: the layer of schmutz may or may not indicate anything of consequence. get a paper towel and wipe some of it off, and have a sniff. if it smells like gear oil, you can be pretty certain that at some point there was either a spill or a leak. that would be enough reason to clean all of it and then keep an eye on it for the next couple hundred miles. if you have more gear oil-scented gluck, it could well be a leak that deserves some investigating.