You can find the rear axle diagram for the 1986 Ford F250 online at places like Ford-Trucks and at Fortification. You can also find these diagrams in the Chilton's manual for that year.
drawing for bearings and seals of front differential axle
how to remove rear axle from 1988 f250 ford
On a 1986 Ford F-250 : I believe " 39 " is a conventional axle ( non limited slip ) with ( 3.55 gearing )
rear axle diagram ford 350 2004
Ford dealer
I am not positive about this but, I believe that this indicates that the axle has a 3.00:1 gear ratio. Check with you local Ford dealer for a definitive answer.
On a 1987 Ford F-250 : I believe axle code " 29 " means that it is a conventional axle ( non limited slip ) with ( 3.55 gears )
3.73 conventional axle ( non - limited slip )
I believe that is 3.50
I was looking at the 1997 Ford F-250 Heavy Duty owners manual and there is a " C 5 " axle code listed 4.10 ratio - limited slip manufacturer - Ford capacity - 6,250 pounds
Sterling 10.25
You can find a 1989 Chevy front axle diagram at most Chevrolet dealerships. Most local libraries will have Chevrolet service manuals with the front axle diagram.