My 4.0L had a squeak (squeal) when the A/C was on. I presumed it was the compressor clutch. I ended up replacing the two idler pulleys, which has resolved the squeal issue. Although one of the factory pulleys was smaller than the other, I used Gates P/N 38042 for both. Works fine now.
'96 ZJ, 4.0L, 42RE, NP242, stock D30 and D35-LS w/ 3.73, 4-1/2" lift, 32" BFG ATs, Magnaflow, JKS track bars and disconnects, spare tire carrier, Optima red top, ROLA roof rack, KC SlimLite driving lights, CB and Yaesu ham radios, GPS