Thumping noise?!?

[todds3535]

Wondering if anyone can help me out with this one...I have an '87 K5 Blazer that I bought about 8 months ago, when I first bought it I checked it out as much as I could and the 4 wheel drive seemed to work ok. I really havn't needed it until a couple weeks ago when we got dumped on with about 6-8 inches of snow. So I put it in 4 wheel and take off from a stop sign and get this loud thumping from what sounds like the right front passenger floorboard.
I first thought it was the wheel hopping, but I can't be sure.
After disengaging 4 wheel drive it went away and I brought the truck back home and checked it out, nothing seemed loose or anything. But I havn't driven it in 4 wheel since this happened.

Any help is appreciated and thanks in advance.

Todd