Good advice here. One thing that hasn't been said yet - it helps to moderate your speed in the snow by gearing down, versus hitting the brakes.
Obviously if you need to stop, you need to stop. Hit the brake. But if you are just trying to slow down or keep from going faster, using the transmission as a brake is effective. The brakes are friction devices whose ultimate goal is to lock up the wheels, except the vehicle's momentum keeps them from actually doing that up to the point where it doesn't. The transmission on the other hand is not trying to forestall the rotation of the wheels and can be used to safely slow you down with minimal risk that you lose traction. This is doubly true if you are moving downhill.
A downshift and gently applied brake pressure are identical to the physics at play. BUT, downshifts have fewer safety guards built in should traction be lost.
If you are, say, going downhill and you want to slowdown in very icy/snowy conditions braking has two advantages
1) you can IMMEDIATELY take your foot off the brake if even a light touch locks them up 2) IF the brakes lockup they are IMMEDIATELY mitigated via ABS
Slowing/braking via transmission is problematic because when you downshift your wheels could IMMEDIATELY start sliding/skidding if they don't have traction (trans wants tires to go slower and truck is moving faster. traction isn't enough for tires to bite and slow vehicle). If that happens you cannot upshift immediately meaning you better pray you don't get sideways (which, if you had braked ABD and ESP would work on for you) while you shift and wait for the the tranny to decide when it's going to move into the higher gear (even in manual mode trans shifts aren't instant).
Just my .02 but in icy conditions you are really risking an issue by throwing tranny into a lower gear. I'll test brakes and gently apply much more control IMO. Plenty of traction? I'm right there with you, throw trans into the next gear down to keep speed from creeping back up.
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