propshaft?

[Vincent Smit <vsmit@mengibar.com>]

During the last week I have noticed a slight thumping noise and feel
directly under the driver's side floorboard.  I looked under the truck
and everything is tight and clean.  Is this the front prop shaft's
u-joints?  The thump is not very audible and tends to happen during
initial acceleration and when turning the steering wheel (any
correlation?).  My front prop shaft is blue, is this the stock color?
Will I cause any more harm by driving the truck with this condition?
One last question, what is involved in changing/inspecting the oil in
the transfer case?

Thanks in advance

Vince