Not a significant gain in mileage with the part-time kit but you also have a little less wear and tear on the front axle components. DO NOT get the 16% overdrive option with the part-time kit. It can really make a rig sluggish an effectively lowers a rigs crawl ratio. It sounds as if your current QT has a bad chain. You can drain the fluid and open the inspection plug on the bottom of the case and use a screwdriver to see how much play is in the chain. If it's over 3/4", time for a new chain. Hopefully, you won't need a new low range mainshaft. These are not available anymore, even with a part-time kit. Most of the time the clunks are sign of a stretched chain.
73 Jeep J4000 360 .040 over, Summit K8600 Cam, Edelbrock Manifold and 750CFM 4V TH400/1339 QT
78 Cherokee Chief- Hunting Property Rig
05 Grand Cherokee- 3.7 V6 5 speed auto