It is a fact that AMC did not offer twin grip as an option for AMC Eagles. The factory considered the front axle more traction than you could need.
The twin grip is also very rare on Spirits and Concords, although the option from the factory did exist.
I am putting a factory posi from a 2wd commanche into my Eagle when I upgrade the axle to a Dana44. It will have a drastic improvement in traction and will not affect the vehicle's handling. A "soft locker" is much more aggressive than the limited slip posi, those will have a noticeable jerk to the rear as you make tight corners. For an Eagle set up for rock crawling it would make sense to put a soft locker in the front axle, but a disconnect must be retained for street driving. A soft locker in the front will make a huge impact on steering and road driving.
The viscous coupling on the Eagle transfer case wears out and is supposed to be routinely replaced. None of us do that. Once it wears out your transfer case is an open differential. If you put the NP231 from a Cherokee in, you'd have low range and transfer case outputs rigidly connected. Thats the definition of the term "Command-trac." There are also alot of Cherokees with "Select-trac," which has an open differential or viscous coupling in the transfer case. The impact of lockers in the front and rear will be more noticeable with a solid transfer case.
All Jeep transfer cases, regardless if they have a viscous coupling or not, have directly connected outputs while in low range.
-Dave Spillane-