I just went to change the gear oil on the 1975 f-100 4X4 and found out there is no drain plug. The bolts to drop the plate face the driveshaft and the unit looks like I have to drop the driveshaft and some other weird looking contraption with 5 bolts before I can access the plate. Am I doing something wrong or what am I missing. The tag on the rear end reads as far as I can make it out -
WDM AK 4KD
3 50 9 8(or 3 I cant really tell) 2(I think)6A.

HELP!!! This was supposed to be fairly easy and probably is, but not sure of what I have here.

 

Most Ford 9" rears don't have drain plugs. I've never tried this but you're supposed to be able to remove one of the lower nuts and studs and drain it that way. Another way is to use a suction gun and suck the oil out through the fill hole.
It sounds like you've never worked on a rear end with a removable center section. There isn't a removable cover on these. The complete differential comes out of the axle housing. That "weird looking contraption with five bolts" is the pinion support plate, this is something that is unique to Fords for the automotive rears (Heavy duty trucks have the same feature).
Trying to remove either of these to drain the oil would be a lot more work than necessary.

 

I just bought the truck and checked the level only to find it is to low to feel. It looks as though it has been leaking around the seal so I have already bought the new gasket and the gear oil. I just am not sure how to go about it. I guess I need to drop the driveshaft and start from there? I have never done that so its time to learn.

 

WDM AK: according to my sometimes-wrong decoder ring:

F100 '73-'78, 3.50 ratio, 9" ring gear, non-limited slip, 31 spline axles.

The "3 50..." also confirms 3.50 ratio, non-limited slip, as it would read "3L50..." if limited slip.

Try pulling the ONE lowest bolt off the pinion cover, it might drain some oil if you build a fire under it first. Otherwise, the suction method or pulling the whole pumpkin is the way.

 

Whats the difference between a limited slip and non limited slip?
Since I am pretty sure the rear end is slowly leaking around the gasket I am going to have to replace it and replace the gear oil. Since it is not what I expected what do I need to do? Differentials for dummies if you will. I have never done this. The most I have done is the tune-ups, oil changes, alternators, heater cores, things like that.

 

You start by setting it on jack stands, then you remove the wheels and brake drums, now you are looking at the axle it will have a large hole inbetween two of the studs for a socket to fit thru. You rotate the axle to remove the 4 bolts on the bearing retainer then pull the axle both sides. You now drop the driveshaft and remove the bolts to pull the pumpkin if you need to reseal it. If you only want to change the oil drive it to warm the oil then remove a bottom bolt if you have to keep sticking a small screwdriver in the hole until it all drains. You need to make sure the breather is open that is what causes the gasket to start keaking a lot of the time.

 

I have done it before by loosening the bolts around the pumpkin and prying it apart a little bit, stuff a screwdriver in between at the bottom and let ir run out until it stops. They usually seal back up when tightened down but not always. Just have to clean it up when finished and give it a little drive time to see if it did or not.