You either need a shifter sheet replaced under the shifter ball , or your sliding collar is shot on one side . If you've been running the 2lo mod , with just the center ball removed , that will do a lot of wear to the collar and cause them to pop out of gear ....
Sarge, you're noting the cogg/collar that will disengauge and engauge the front driveline in a worn state will cause the truck to magically slip out of 4x4 mode into 2wd mode?
Hmph. I've had that happen to me like 2~3 times.
Don't forget to check your oil levels...mine can tell me when its getting low by popping out...it is the only time it does it...also you might have a tweeked t-case arm casing binding under load, torn t-case mount...lots of things I believe.