"off topic" 4 wheel drive problem

OK, don't shoot, but it's not even a Mopar .. It's a '95 F250 that I've been plowing our yard out with for 10 years..

After plowing today I noticed a noise I never heard when taking off from a dead stop in 4 wheel drive .. It's kind of a clunk clunk ....clunk clunk ... clunk clunk... that increases in tempo with speed ..

I tried un-locking the hubs and leaving it in 4 wheel and it doesn't do it.. It doesn't do it in 2 wheel drive either, only in 4 high or low..

Anyone got any ideas ?? I don't need major headaches with this old girl right now, but I'm guessing something is F'd in the front diff??

thanks in advance,

Kenny