As near as I can measure just right at 20 7/8"
As an aside all roll up glass was interchangable between front and rear doors, at least up until the tape regulators (I don't know about those).
J-trucks, wags, and cherokees all used the same roll up glass.
ASE Master Collision Tech
"Beast" - 81 "S" W/T, 85-360, T-18, Lock-rite,wag alloys, 31-10.50 Pro Comp MT's, Warn 8274 in a fabbed bumper/deer strainer(tested and approved)
88 XJ, 3" lift, 31-10.50's, custom bumpers and winch/tow bar mounts, Warn 9500HS, custom sliders/steps, camo paint, & headliner
Member: FSJ Prissy Restoration Assoc.