I have a 1994 mazda b 4000 with the dana 35 ttb front end. I went through some mud the other day and now the front end locks up in 4x4. The front diff. has been leaking for a little while but i make sure it always has gear oil in it. In 4x4 it makes a grinding/squeaking sound then it jerks to a stop. I can power through it but it just does it again. I cant make it more than five feet without it locking up. What could cause this?