4wd+throttle+30mph=Shaking front end

[tykesplace]

Maybe this has happened to one of you guys.
Usually I just use to 4wd when the roads are snowy until I get going, but the other day conditions were pretty slick so I left it in 4wd. At about 30mph the steering wheel started to vibrate pretty severely. But only when I was pressing on the throttle. Let off and the shake goes away. Touch the throttle while in 4wd over 30mph and it happens every time. Switch off 4wd with the throttle on and the shake persists until I let off the throttle. It doesn't happen in 2wd at any speed. I haven't tried it on dry pavement. Only snow and ice.
Truck details in sig. Stock tire size, no lift or other suspension mods.
I'm thinking unbalanced front drive shaft, but at only 30mph? I'm nervous to take it faster to see if the shake goes away at higher speed. I had the truck checked out before I bought it last month and the tech said the front bearings and ball joints were fine. Any other thoughts? Its too freakin' cold right now to take out the front drive shaft, but I think it might be worth trying. There's a local driveline shop that will check balancing for free.
Whatcha think? TIA!
Darrel

[mschn99]

it will be one of two things more than likely......

either a frozen front axle u joint....or a bad u joint on the front driveshaft. The rear joint on the front shaft is a double cardigan (spelling...lol) joint and they are a pain to change....half the time you ruin it. take the front driveline out and inspect the joints for binding....also visually inspect the axle u joints and if the front driveshaft doesnt prove anything then you may need to remove the axles and inspect those closer.....

[pipelayerjdh]

I think Marc hit that one. The reason it did not stop until you let off the throttle after switching to two wheel, is the hubs don't disengage until you let off.

[tykesplace]

Sounds like good places to start. Thought of another question. I'm thinking that out of 4wd, things up front are freewheeling and so no damage should ensue before fixing this? i.e. can I safely wait until its warmer weather?