To recheck what I previously posted, I pulled my dipstick this morning and it is about 14" long. The problem with that dipstick is that it sits just above the rear PTO shaft, only slightly to the right. If you are lucky, it is laying in the bottom of the rear differential/PTO casing. The closest access I can think of is the panel blank where the Mid-PTO is installed. If you drain the hydraulic reservoir and remove either the blank or the mid-PTO (if installed) you might be able to fish out the end of the dipstick.
EDIT: I checked some diagrams and the bottom of the gearbox where the rear PTO sits is isolated from the mid-PTO by a casting. It is NOT possible to fish from the mid-PTO blank. It appears the only way to gain access to this area is by removing the rear PTO shaft or the the complete 3PH hydraulic assy from on top of the differential. The good news is that unless you have a mid-PTO installed, the dipstick probably dropped past the rear PTO shaft and is laying harmlessly in the bottom of the reservoir. I'd be tempted to just leave it and hope for the best.