Ditto on the other two posts. Most people are used to the older N's with only outter axle seals and forget that the later N's have 2 axle seals. Also. if the axle is groved.. try to sleave it.. or at least braze it up then turn it down smooth. Some even use jbweld with great success.
Also.. really sound slike you are running with way too much oil in the rear end. It is only made to use 5 gallons anyway, and most people overfill. The 'accepted' method is to remove a lower bolt on the round inspection cover, and only fill till oil comes up to that level. Probably about 4.5g's. This keeps oil from pooling at the seals in the axle, and the tranny input shaft seal.
Also.. if you are using the new style UTF / 134d oil.. you may want to consider going to a 90wt gl-1 or gl-3 oil. 90wt gl-1 is available at many wallmarts, and at napa as part # 65-205.. Also.. TSC sells a 'ford tranny fluid for tractors 39-52'.. this is widely believed to be very close to the oem 90wt fluid, and to be mildly ep.. like the new 8n manula specified.. perhaps like a gl-3.
The thicker oil is less prone to escape weak seals, and also may help the lift be a tad stronger and knock less. In cold weather it will need a few minutes to warm up.. and I reccomend you depress the clutch when starting to prevent the starter having to drag over the tranny in that thick oil.
Soundguy