Bottom line, there is an axle end play issue with TrueTrac in some Dana 35c units, and a pretty simple resolution that will allow it to work properly.
Link broken, so here is the write up:
TrueTrac problem & solution for Dana 35c
Thinking about installing a TrueTrac differential in your Dana 35c rear axle? Then you and your installer should be aware that you may experience a fit problem on this particular axle. If your installer is a competent welder and prepared for the problem, then the fix is simple and adds about an hour of install time. In short, you may need to build up the ends of your axle shafts slightly by welding in order to take up excessive endplay. I know because I just had this install completed, and I experienced the problem.
In its OEM configuration, axle travel in the Dana 35c is limited in the outward direction by c-clips; in the inward direction by the spider gear cross pin. The TrueTrac unit replaces the OEM carrier and spider gears with a helical gear-driven unit. This unit does not have spider gears, and therefore does not feature a cross pin in its design. Without a cross pin, inward travel of the axles must be limited somehow. So, for the Dana 35c, the TrueTrac features a pin-like spacer to limit inward axle travel.
The problem is that Dana 35c axle production spans several years, the axle is used in different applications, and there are at least two different cross pin diameters specified, perhaps related to duty or perhaps related to a global design change, I'm not sure. Yet the TrueTrac unit for this axle is provided with a single spacer pin of a size corresponding to one of the various Dana 35c cross pin diameters.
If the diameter of this pin-like spacer is identical to your OEM cross pin, then all is well, or, at least, your axle endplay should be unchanged.
In my case, the diameter of the TrueTrac spacer is smaller than the cross pin it replaces, so my axle endplay increased by half the difference in the diameters.
My installer, a qualified Jeep mechanic, measured axle end play 0.016 on one axle, and 0.014 on the other before removing the OEM carrier. For the record, he also measured the OEM cross pin diameter at 0.686 inches. We somehow neglected to note the TrueTrac spacer pin diameter, but I will measure it again next time the cover is off (it could be deduced from the other measurements, but I would prefer to report a direct measurement of the spacer pin).
After installing the TrueTrac unit 912A569, he measured axle endplay again, finding it to be about 0.030 on both sides. He explained that a certain amount of endplay or space between the ends of the axle shafts and the cross pin is desirable for thermal expansion, lubrication, etc., but that 0.030 each side was too much, would lead to premature wear somewhere in the assembly, and could probably be felt or heard in cornering. So we decided to build up the ends of each axle slightly by welding, then grind or file back to acceptable tolerance. He did this, achieving final clearances of about 0.006 each side. This required about one hour of extra work.
Axle end play can be measured with the TrueTrac unit in place, so those users who have already installed this device, but are now concerned that they may have excess endplay, could take measurements the next time the differential cover is off.
Ron's Machining Service 1-800-694-3098 deserves special recognition for advising me of this potential problem when I called to order my TrueTrac unit. No other vendor that I spoke with was aware of the problem, even when I asked directly if there was any known problem with a Dana 35c fit. Ron put a sale at risk to advise me of the endplay issue, and then he phoned Eaton (manufacturer of the TrueTrac), as I did, to learn more. Since Ron had the right combination of price, service, knowledge and integrity, I was happy to give him my business. He also shipped quickly - I had my products in less than a week.
Finally, the product manager at Eaton who returned my call was friendly, explained the problem clearly, and suggested the axle build-up solution.
Eaton should provide a choice of spacer pins in the kit with this product. In the meantime, perhaps these notes will help others anticipate this relatively small problem and achieve a better quality installation of the otherwise fine TrueTrac product.