I have a 98 chevy half ton truck 2wd and was wondering if a 98 chevy half ton truck 4wd differential will bolt right up without any modifications, such as universal or spring dimension?  The reason I am asking is that I tow a trailer and need the 6 bolt 16" wheels (tire problems on the interstates) and a higher # gear ratio that comes with the 4wd differential. The differential I have now is a 10 bolt 3.08 gears.  Also will a 3/4 ton 2wd possibly fit?? Any help will be appreciated......  

Your better option is the 2WD 3/4 ton rear if you need more gear and strength for towing. You'll probably need to change the leaf springs, but the rear will be the same overall width of the 10-bolt. I'd also grab the driveshaft out of the 3/4-ton truck you get the rear out of, a longer pinion will probably mean a shorter driveshaft.

Another option for you, look at the 10-bolt out of a '99 - '03 1/2-ton Chevy/GMC truck. These 10-bolts had the six lug wheels, and a gear swap is pretty easy for a better towing gear.  

I put air bags above the rear springs to get a much stiffer suspension (that is why I now have tire problems----can only get load range c tires with 15" rims).  I didn't have tire problems until then but I knew I needed stiffer suspension.  Didn't know either at the time that no one makes range d tires for 15" wheels.

Would the spring centers be the same on the '98 3/4 ton rear and would you know if the spring centers are the same on the 99-03 and would the driveshaft fit the 98.  I have an extended cab with the two piece driveshaft.

thanks.