When you engage the left turn signal, you get the left front to flash, but BOTH rear signal bulbs flash. Is that correct?
If so, its almost certainly your "multifunction switch", which is what controls both the turn signals and 4-way flasher. My description above is a common failure mode. A new switch should fix the issue.
1992 XJ 2" lift -- RIP
2000 XJ 3" lift -- Australian Ford Big Brakes up front, rock rails, ZJ rear discs (with WORKING parking brake), C4x4 rear bumper/tire carrier, Hella H4 headlights, IPF driving lights, OEM skid plates, OME springs/shocks, JKS LCA's, JKS Sway Bar links.