Re: 01 exp differential problems??
I'd take the rear diff cover off and give the pumpkin a good inspection. Check the magnet for any debris. OE limited slips (if you are so equipped) are not known for their durability. If the diff checks out fine get the U joints and CV joints checked. Last thing to check (you can do this yourself) is to check your wheel bearings. Jack up each wheel until its a couple inches off the ground. Put one hand on top and another below the wheel and try to rock the wheel towards you. If the wheel has play you know that you have a bad bearing. While you change the rear diff oil, change the front as well (space will be minimal and you might require a special siphon tube to put new oil in).
If you haven't had a transfer case service in a while it'd be wise to do that and get the shop to inspect the chains/belts.
When you go to a shop that you are not familiar with, be firm and up front; ask them what their policy is if they cannot fix the problem after you've spent money knowing that it would be fixed. If the truck requires more diagnosis time make sure they give you a discounted hourly rate. They might tell you that they follow a 'book' value, but most will give in for the added business.
To engage into 4wheel lo be sure to place the transmission in neutral first. If it will not engage at all, put the truck in reverse, and then neutral then try 4LO. As trucks age the transfer case can become somewhat worn. Worst case is that a band/chain is bent or worn out a bit and will require the Xfer case to be rebuilt.
'00 Durango R/T 360ci 290hp (modded); 138,500m
'06 Pontiac G6 GT 3.5L 220hp; 44,000m
'12 Chrysler 200 Limited 3.6L 283hp; 13,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!