I have an 81 f100 light duty 4700 pound gvw it has a 9 inch drop out rear end in our trucks did they still use the 28 spline axle and the31 spline axle or did they finaly get smart and go with the 31 spline only

 

I don't know about the f100's, but the f150's all went to 31 spline axles. Why do you want to know? The 28 spline is a good axle too.

 

im having viberations when I put it in 4th gear the kind that will shake grannies dentures out I have ruled the u joints they are both tight while I had the driveline out I turned the rear yoke left and right and there a 1/16th play before it catches on the splines on the rear yoke

 

The 28 or 31 are the axle splines, not the yoke splines. Your best plan is just find another complete rearend and bolt it in place. Here are some places is WA that have one.

2.75 to 1

1981
Rear Axle
Ford Truck 100 2.75 15694 $Call Pasco Auto Wrecking USA-WA(Pasco) E-mail 800-572-9624

3.00 to 1

1981
Rear Axle
Ford Truck F100 4-1/2' BC (10' br), 3.00 ratio CHECK RATIO 208,000 C K05338 $225 Gerry's Foreign Auto Parts USA-WA(Kittitas) Request_Quote 1-800-848-5578 Request_Insurance_Quote
1981
Rear Axle
Ford Truck F100 4.9,AT,CK.SIZE,VERIFY RATIO 28128 $Call Pasco Auto Wrecking USA-WA(Pasco) E-mail 800-572-9624
1980
Rear Axle
Ford Truck 100 *CHECK 20256 $Call Pasco Auto Wrecking USA-WA(Pasco) E-mail 800-572-9624

3.25 to 1

1981
Rear Axle
Ford Truck F100 VERIFY RATIO,NO TAG N6877 $225 All West Coast Auto Wrecking USA-WA(Shelton) Request_Quote 1-800-720-4599 Request_Insurance_Quote
1981
Rear Axle
Ford Truck F100 4.9L,4Sp,RWD,CUSTOM 140353 $Call Nisqually Auto Wrecking USA-WA(Lacey) E-mail 360-456-6222

All the above are rearends with the small bolt pattern. I don't know what you have on your truck. I also do not know what you have planned for this truck, but you can use this as a opportunity to make this truck whatever you want it to be(larger bolt pattern, 4x4, etc.)

 

your right I definitely need to figure out what I want this truck to be but i was thinking change out the 3rd member rather than having to change just the yoke and have to deal with shims and pinion depths .that's why I asked about the axle splines I know in the 70s they used both and you had to know witch one you had. it turns out it was a tune up issue one plug was fouled out and not firing and the other was a burned plug wire and I thank you for your help