I've tried before to post. ??? I would look at replacing the oil pump, if on a tight budget. Also, you could inspect the crankshaft bearings and journals. Also, check the rod bearings.
Ok, finally, got to post again. As far as a rebuild, I could find some kits for the RENIX. Cleggs, Titan and some on Ebay. I think most are Engine tech. I did see one on Ebay with Sealed power pistons. I see Engine Tech, Mahle and Sealed Power pistons on Rock Auto. It shows those cover from 87-95 for XJs. The ones for 96-2006 4.0L seemed to be Hyper-Eutectic, and a bit higher compression height.
I might look at piecing together the parts. Mainly for quality. Especially, for lifters. I'd get ones made in the US. Johnson Hylift are suppose to be VG. As for a camshaft, I'd try and get one with a split lift. A bit more lift on the exhuast cam lobe. Over .450 lift, you might need aftermarket springs. I am having Schneider's regrind some 99+ stock camshafts. .450 Intake, .452 exhuast and 252 duration.
Also, I am porting some heads. They will have Chevy valves. If I did it again, I'd check out Russ Pottengers Chevy LS valve kit. I think $300.