2001 Blazer LS ZR 3-button on dash, 5-spd. when towing snowmobile trailer up hills front end pops out briefly- does not sound good at all, had to put into 4-low to continue up hill. any ideas where to start?

 

Ill take a guess and say its probably your front axle disengaging when under load. The front axle is engaged via a vacuum actuator under the battery tray which can go bad and cause an issue like this. There is also a cable that could be kinked or damaged that goes to the axle tube itself. I would grab a buddy and get under the truck to see if this cable is engaging properly when you push 4wd. A lot of people use the "posi-lok" manual locking cable instead of repairing the actuator, I plan on installing one if/when i have issues.

 

A plain vacuum leak could be causing the problem also, like KM346 said, the axle engages via vacuum so if you have a leak somewhere, it will pop out because the vacuum to the actuator goes away