Re: 00 Navigator Thumping and Not Wanting To Go...
First, AWD and 4x4 are commenly mislabeled.
AWD is essentially a full-time system, where both axles are turning, but at independent speeds so as not to bind the axles and cause damage while driving on pavement. 4x4 is more of a selectable system, which you have as you mentioned a 2wd option. 4WD isn't AWD. There can be up to two settings on newer 4WD vehicles, the most common being 4HI and 4LO, 4HI locks both the front and rear axle together for better traction only on slippery surfaces. If you go off road, 4LO is the one to use, as it effectively lowers the crawl speed by up to 3:1.
Some 4wd systems have both a 2WD, and 4FT (AWD) setting. 4FT can be used on pavement, but allows the axles to turn independently of each other so they don't damage the gears. On some earlier trucks (not Ford) you could get 4LO-N-4HI-4FT-2HI as an example. It explains each mode in the owners manual.
Now, the tires. You can run different diameter tires just fine with an AWD vehicle. There are some exceptions to this rule, AUDI being one, as their system takes into account several factors such as traction control, electric braking, etc, which can throw off the PCM and fool it to believe that there's a loss in traction when one axle doesn't have the bite (think bald tires) than the other axle does. This can eventually damage the system.
You don't have to worry. I have an AWD Durango myself. You can run 30" tires in the front and 31" tires in the back and not worry. Why? Because the axles turn independently of each other in AWD. When you lock the axles together (4HI), then it can be a problem, but only if you do this on pavement or another hard surface. On loose gravel or dirt roads, it won't affect anything as each axle can move and adjust itself as necessary. Besides, if you ever had to use your spare tire, which is a different diameter, why bother having one if the shop says you can't use it! It's BS. I do recommend on rotating the tires regularly, as it helps promote even wear.
'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!