I've got a D70HD under the back of my TJ.
Some other info on D70s:
The D70HD uses larger carrier bearings and has 4" dia axle tubes.
The D70U has a SMOOTH bottom... very nice.
Uses the same D60 pinion yokes... interchangability is nice. since you've likely got a 60 in the front(like I do).
Deeper gears available(down to 7.17) as already mentioned... I LOVE my 5.86s with stock 4.0
Not sure about 14 bolts, but most D70s have equal length axle shafts left and right... less spares.
The housing is cast steel... very easy to weld vs the 14 bolt's cast IRON center section.
1.5" diameter shafts... mine are 35 spline. There are some oddball coarse splined axles out there too but 35 spline is relatively common AFAIK.
Carriers... kinda expensive for this axle but SOME of the D70 carriers are 2pc carriers... in which case the Detroit upgrade is actually relatively cheap IIRC. Some have 1pc carriers and you need to buy a full Detroit then as well.
Another thing on gears... there is NO 5.38 gear ratio available for any D70... so if you have a D60 for the front or something already geared to 5.38s... you're better off just getting a 14 bolt instead. Similar strength but you won't be re-regearing your front axle. I found this out when the previous owner told me it had 5.38 gears... I was very happy about that... then found out that 5.38s don't exist... inspection found I have 5.86s which work really well on 40s. Tons of power and it's not too terrible shifting all the time.
