Alright, so my diffs are pretty bad. Each driveshaft has like an inch of rotational play. I decided to take a look at the rear one today, and having never done diff work before, I didn't realize that you needed to take the axles off. So I didn't actually get a look at the gears, but there was alot of metal shavings on the plug, so they're very likely on their way out.

I've read on here that usually you can go up to 31s on 4.10 gears. Right now I've got 75s (30"s)? And I would like to go a bit bigger on that. However I definitely don't have the money right now for a full set of tires and gears. My truck is an 81 and I've read that this year and this year ONLY had 3.90 gears in it. My truck pulls around 3k-4k RPM in 5th at 80 mph.

Through my (possibly wrong) calculations, if I did 4k rpm at 80 mph now then I'd do almost 5k rpm on 4.88s. I can't remember exactly but I think I was pulling 4-5k rpms in 4th when I went 80. I know I didn't go past 5k but I can't remember the exact number. However the only time I go 80 is on interstate and I don't do that very often.

Also, there's the problem of my driveshaft not going back on. It came off just fine, but now it won't go in on the front part far enough to where I can get it hooked up to the diff. Is there a certain way it goes on the splines? I didn't see a key or anything. I kinda need my truck for this weekend, so I'd like to get the driveshaft put back on.

 

I hate it when my shaft won't go in.... sorry, I couldn't resist!

On another note, what exactly is your question here? It's not very clear.
If you're just asking about the splines, they should line up with no problem. You may have dirt in there keeping it from sliding in properly.

 

Oh yeah my question... Should I just put in 4.10s, or should I go to 4.56 or 4.88 in anticipation of larger tires?

 

4.56's with 31's might be a little too torquey and you will def lose your highway speed

 

I have stock 4.10 gears in my 83 and it turns my 31x10.5 tires just fine. Or at least it did when the motor was in it. Right now It's a flintstone mobile until I finish this rebuild. I was able to drive highway speeds (65-75) with no problems on a slightly tired motor.

 

And I'm not completely sure what's going on with your driveshaft...you took it apart? Or just took it off? I've never let the two peices slide apart, so I don't have any advice there. I'm guessing that's what you're talking about, since it's pretty obvious how it connects back to the rear diff and t-case. It is two piece, so it will compress and expand, you know?

Are you talking front or rear? Long bed or short?

 

It turns out the driveshaft can only go back together one way. But there are no markings or the like showing you which way, so I had to take it off and put it back on rotating it one spline at a time. I think I'm going to just get 4.10s for now. Hopefully it will last me until I get new gears.

 

Yeah, there's no reason to take the driveshaft apart, unless you're specifically fixing something in there. Just take it off the truck as one piece.

 

If your going to spend the money for 4:10 gears buy the 4:88s. I run 31" tires normally but I also run 33" at times. I turn about 3800-3900 RPM at 75-80 MPH with the 31" tires. I was just at the Carlisle, PA Truck Nationals and got about 20 MPG running 70-75 MPH with the 31" tires.

 

Oh well this is alot of what I was looking for. Thanks man now I just have to find someone who has experience with differentials...

 

Not true. You just didn't try hard enough. So, in actuality, you really have no idea whether you put it back together the right way or not. Since you're probably completely unaware of what the right way would be. Keyword: Phasing
That you do. Because at this point you're lost deep in the woods.

Keep at 'er though. You'll find your way out eventually.

Hint: Trucks that came from the factory with the 31" tire option had 4.56s. Trucks came with 33" tires from the factory had 4.88s.

 

I had 5.29s on my truck running both 33s and 31s. Definitely had some torque and I had no problems with it at all on the highway around town in traffic. Didn't take it out of the city much so I don't remember how much the top end would suffer if you're doing like 70-80 MPH.

 

MH hit the nail on the head on d/s phasing

I use this gear Calculator all the time. Put tire size and gears etc..gives you rpms at what ever rpms..
http://www.grimmjeeper.com/gears.html

 

Yep... mudhippy was right! Anytime I take the drive shaft apart, I mark it with nail polish, that way I can line it up, and put it back exactly how it came apart. I do the same when replacing u-joints. Many times in the past I didnt do that, and had to spend countless hours trying to figure out why I had drive line vibes, with brand new u-joints. Its especially important to do this when replacing u-joints on rigs with a lot of miles... after a while, the shaft kinda wears in a certain way, and putting it together wrong will throw it out of balance.

 

If you havn't switched gears yet. Not all 81s had 3.90 gears. My 81 was manufactured in late 80 and had 4.10s. So check your manufacture date and also check the build info on the door jam. If its g31 then it's 3.90s, of it's g29 then it's already 4.10s (there's 3 numbers not 2 but the 3rd doesn't matter for this application.)