1989 c2500

was wondering what rear axle is under it. and how might i lock the rear. i have a 60 front that i am going to put under it. will the stock rear be beefy enought to handle big tires?  

You need to see if the rear axle is a full floating unit or not.  Easiest way to check is to see if the rear axle has what looks like a hub on it or not.  Should be a hub with a flange that is held on by 8 bolts.  You may have to pull the wheels and tires if your rims have the center caps on as they will cover it.  If it looks flat, like a half ton pickup then it is a semi float rear end.  If it is the full float unit, then you are golden and have nothing to worry about.  That is the 14 bolt FF that so many guys swap into their trucks.  Put the 60 in and get a t-case and you're wheeling.  If it is the semi float, no big trouble as you can find a 14 bolt ff out of any 3/4 ton or srw 1 ton pickup of your body style and it will bolt right in.  The semi float is not worth keeping if you're putting a 60 in the front.  

i know it is a 14bolt, but not sure of the ff semi floater.  

Like I said, easiest way to tell is to check and see if you can see the end of the axleshafts bolted to the hubs.