1500HD Transmission/T-case - Can of Worms

[Alex V.]

My FIL's '02 1500HD has developed some pretty strange drivetrain behavior the last few weeks (after he drove it in the snow storm here in central KY) and I've undertaken to look into the truck.

Symptoms:

Has mentionable ATF leak, but has been more or less kept an eye on.

(Call this a preexisting condition) Occasionally, intermittently goes into limp mode - seemingly more likely to do so in snowy, muddy, or other conditions where traction is compromised. Can be cured by cycling the ignition several times.

Since the storm a few weeks ago, random shuddering (like the tranny clutches are slipping) while cruising - not during or immediately following a shift, TC lockup, etc. Let off the gas and it quits doing it.

The last time it was parked (in 4-LO), upon next startup it refused to go into reverse and the engine seemed to be under constant load, not wanting to rev past 2,500-3,000 RPM. BUT when he fired it up to load it onto the trailer he stomped the gas once to demonstrate it for me, stomped it again, something audibly popped under the truck, after which reverse and normal throttle response were back. It then functioned normally to the end of his driveway (1.5 mi.) and again backing off the trailer into my shop.

Took it by Advanced Auto Parts and had them pull codes - all 19 of them.
(Listed in order)
Stop Lamp Switch C0281
Electric Brake Control Module C0265
Left Front Speed Sensor Missing C0226
Rear Wheel Speed Sensor Erratic C0237
Right Front Speed Signal Erratic C0223
Class 2 Data Link Malfunction U1000
Rear Propeller Shaft Signal Circuit Malfunction C0300
Front Propeller Shaft Speed Sensor Malfunction C0305
Motor A-B Circuit Malfunction C0306
Transfer Case Lock Circuit Malfunction C0321
Encoder Circuit History C0327
4-LO Discrete Output Circuit Malfunction C0359
General System C0374
Front Axle Circuit Malfunction C0379
ECU Malfunction C0500
Transfer Case Neutral Indicator Circuit B0790
Mode Switch Circuit Malfunction B2725
Class 2 Data Link High U1301
O2 Sensor No Activity Detected Bank 2 Sensor 2 P0160 (Realize this probably isn't related.)

He was concerned the transmission was toast, but fluid level is currently good and it doesn't smell bad at all. I haven't pulled the pan yet. 220,000 miles but it was bought at a farm auction with a salvage title so its history is mostly unknown.

What pertinent information I've found so far suggests it's most likely a lack of ground, bad wiring/connectors, bad sensors/switches, or bad T-case control module, but I don't have my own scan tool nor much in-depth knowledge of the fine points of how this system works. Though I can certainly see why it would limp and give the transmission fits if the signal from all the driveshaft and wheel speed sensors is being impeded or processed improperly by a faulty computer. Thoughts?