So I'm putting a 3.9 diff into my 91 car and I have the diff all bolted up and the drive shaft connected to the transmission but I can't get the CV axles to fully seat into the differential itself. What should I do?

 

Disconnect the control arms from the hub assembly. Have one person hold the axle and keep it aligned to go into the diff and make sure you don't scratch the smooth axle surface. Push from the hub end and it should pop right in. If not, try a wood block on in between the wheel studs and tap it with a rubber mallet.

I tried doing this with the hub assembly still connected to one of the control arms and you just can't get the leverage needed to get it into the diff.

 

I second LukeG, you need to whap it in to overcome the spring clip pressure, just as he describes.

 

Had the same few days ago. Rubber mallet on the axle shaft out of the hub and upright taken off.

 

Axle, hub and upright all assembled together. Put the axle end into the differential and the long bolt through the bottom of the upright, put the nut on the end, but don’t tighten it down. Leave the short bolt out of the top of the upright. Get a jack under the lower control arm and lift it up until the axle is lined up completely straight with the hole in the differential. Now swing the whole hub assembly around the loose lower bolt to ram the axle home into the differential.

 

Okay so the swap is complete! It drive nicely but there is a big squeaking noise coming from the back passenger side, any ideas what it could be? Sounds rotational

 

Bit late to this party, but splines slide in really easily and nice, IF lubed (duh, just for the record), AND if the circlips are located correctly. Basically the tangs hang down, and everything orients itself as the spline slides in. No force required. I just did a diff rebuild, and I slid them in one-handed, then assembled the outer into the hub. Always done it this way, and I have done plenty of diff swaps. All on NBs though, but that looks like an NB diff, not the early 1600 diff with two-piece drive shafts.

You can do the hubs first, and use the knuckle for leverage, but that is not necessary, and I would avoid any force that can be avoided where bearings and seals are involved.

Orient them the other way (tangs at top), and the circlip resists the final shove. When you compare the two positions, you can see that the 'tang below' distributes the profile of the circlips more evenly, whereas 'tang at top', the bottom of the circlips is much lower (because gap between tangs).

If you have noise coming from this location, and you used anything but the lightest tap of the rubber mallet, you may have seal and/or bearing damage.

Only you know what you did to get that shaft in - it is now time to come to Jesus, confess and repent your mechanical sins, and start out again. (hopefully though it turns out to be nothing of any consequence)

Good luck