Okey doke: first things first: parts installed under warranty are covered by the 12month/12K mile warranty ~OR~ the remainder of the factory warranty, whichever affords the greater protection. So dig up the RO's from the ones replaced under the PT warranty. If it was less than 12/12 since the last repair regardless of whether the PT warranty has expired, they need to fix it. Now here's a problem: the replacement parts warranty does not 'reset' when a part is replaced, it's always figured from the date of the first repair. If it was 50k, then that expired at 62k. Now, if the second T case was replaced between 62k and 70k, then that one should have its own 12/12 replacement parts warranty. So dig up the RO's and peruse them carefully.
Now. The legal side from the way I have seen it happen a bunch of times with many manufacturers. billy3esq style disclaimer: I ain't no damn lawyer. 
If it's out of the 12/12 and PT both, Chrysler is under no legal obligation to repair it at no charge again. What you would be looking for is a 'goodwill' repair which is up to the representatives of Chrysler Corp. A lawyer can huff and puff all he wants but unless he can prove the part was defective or improperly installed, no such luck. So I would not waste my money on a lawyer just yet.
Why did I say 'just yet'? You said the T case was full of some 'awful crud'. You need to find out beyond any doubt what that crud is. I hope you have a few ounces (preferably a pint) for analysis. Down here there's a place called Bolyn Lubricants which used to do analysis (they might still do it). Pegasus Auto Racing offers oil analysis through a third party, you might check with them. Chrysler spec'ed a synthetic lubricant in many transfer cases (and axles, too) if it's an automatic tranny truck and has a rear seal ('sandwich seal') leaking it may have ATF+4 mixed with the synthetic lube. That may make no difference at all, then again it may be a problem. If the sandwich seal was leaking and it got ATF+4 in the t-case causing a lubricant breakdown or the wrong lube was added at replacement, you might have a case. Or if the t-case was staying engaged at highway speeds, it could have generated a lot of heat, causing lubricant failure etc. In that case there should be a HEAP of metal particles (bronze, steel, etc) in the fluid and he might have a case, meaning that something else could have caused the failure (vacuum switch, bad electric engagement motor, etc).
If it has water or mud or etc in the t-case, your b-i-l is screwed. These vehicles are 'trail rated', not 'pond rated'. BTDT.
