i heard that the 1200 datsuns have weak diffs. if so whats a good easy to install diff.

They are perfectly fine with an A12 engine in front of them, but if you put say a worked A15 in front of it and revved the guts out of it and dropped the clutch all the time it would break.
I have a standard diff in mine with a worked A15 and it has never given a problem, it depends how you want to drive it.
If you want to drag cars off at the lights try installing a 1200 ute diff.
If you look up the tech section to the left of this column, you will find other options.

I gotta go with Sidedraught here. The H145 diferential is not 'weak",... when used in the application for which it was designed, IE, in a stock-ish 1200.
I used one for about 200,000 miles in my 1000 sedan with improved 1200GX & five speed. A lot of the trick is in knowing how to drive, & un-necessarily broken components are a sure pointer to those who dont.

If you plan to use a significantly more powerfull engine, then abuse the crap out of it, you can do what 1200 owners have been diong for decades & use the Datsun 1200 ute rear axle assembly. These will interghange into 1200's & they use the stronger H165 diferential. Suitable ratio diffs can be sourced from 910 Bluebirds & Stanzas.

These 1200 Ute rear axle housings will accept the rear disc brakes from the 910 Bluebird TRX & LX models,... so i believe.

Respect your Datsun,... treat it well.