Hey Julio,
There should be an o-ring on you speedo piece that goes into at the trans (or transfer case for 4x4s) check to see if its leaking past there. Or if you have the electronic speedo, make sure the sender housing isn't cracked. Its made out of plastic and can't take too much of a hit.
Hope this helps, Jones
------------------
Jones
'84 Ramcharger w/CTD, non-gated 12, torqflite, NP205, Dana 60 and 70 w/ 3. 07s. Looks more or less stock, definately doesn't sound stock.