Kestral wrote:My question is is it possible for the truck to be in 4x4 high and jump to 4 low on its own without turning the switch to 4 low?
I'd say NO, it's not possible, especially if there was no indication of 4LO on dashboard.
What I'd check first would be coolant and transmission ATF fluid for cross contamination. Also check ATF level (warmed up, 15 minutes driving, then check level with ATF dipstick when engine is still running). If both fluids are clean and ATF level is appropriate, you can probably breathe easier.
but still better check for DTC (Diagnostic Trouble Codes), either by yourself with some ELM OBD2 adapter from eBay and Android/iPhone, or during visit to your local garage.