A crawler box is like Low gear in an Auto truck, cept its got an insane ratio usually double that of 1st gear on a manual. It's so strong, you can let the clutch out without touching the gas, and roll along at about 2mph all day without any worry about anything... of course, it can't go more than 10-15 mph at max RPM so it's useless for anything except getting a quick start or moving something that just doesn't want to go in a higher gear. You can make 36inch boggers cook with a crawler gear and 3.73 gears on dry pavement... but doing a brake stand in a manual truck is not my idea of a smart thing to do.
My former father-in-law (RIP) had a crawler box on his 454 C30 rig that he could crawl with a strap to a 20 ton tractor... he'd winch it until the rig started to shake, then put it in Low and fire it up... I watched him drag that 20 ton tractor a good 5 feet before he let the winch cable out again and repeated the process... Needless to say, if you have a crawler gear AND 4wLow, you are not going more than 5 MPH at max RPM