There is nothing really wrong with the D70. Axle tech hasnt changed much since GM introduced the full floating rear axle in the 1930s. Bearings and seals and material quality are where the advancements have come from. The D70 has a 10.5" ring gear, just like a corporate 14bolt or Sterling. But its older tech.An axle swap is definately something i want to do as the dana 70hd is kinda an odd ball. Only one option that i could find for a limited slip/posi.
Also im new gearing. But i feel with out even towing that the gearing should be different. Maybe its just the t18. Im set with my new tire size. would that play into choosing different gearing? I think its the stock tire height
I have an np435 in my bronco that is basically the same as your T18. Being basically a three speed plus granny gear, the gear splits are larger than a 5 or 6 speed. This means you cant always keep the rpms right in that 3000 to 3500 sweet spot. With stock tires of about 31 inches, a 4.10 to 4.56 gear would be what i would choose. 4.10s will give you a slightly higher top speed but less grunt.