1994 Explorer 4 wheel drive problem

I have a 1994 Explorer 4 wheel drive auto hubs. The explorer only has 50000 orginal miles, and the front lock out hubs were replaced 500 miles ago. I am still having problems with it. Sometimes when you turn there will be a thumping/grinding sound from the front, and the vehicle will pull to the right. If you stop and back up, it will go away.Like i said the lock out hubs are new at a cost of around $800.00, now the shop that i use tell me it could be the u joint on the right side going bad, and causing the problem.
Has anyone else had this problem, and if so how did you get it fixed.
The explorer has been pretty good, and has very low miles so would like to keep it.
When you would push the four wheel drive button while this is doing it it works fine.
Thanks for any information

dah

could be u-joints, although its usually pretty obvious. Although there is no load on the front u-joints, so they take a long time to fail. Last time I did a u-joint on one of those, it had over 200K miles on it. I would expect vibration on the freeway if it was that bad

Its either in the auto hubs (incorrectly assembled) or its the engagement solenoid.....but its quite easy to incorrectly assemble those auto hubs. Why were the replaced anyway? Only time I've seen them fail was when the bearings were so bad the wheel almost fell off