As Bowtie said, just turn up the stock torsion keys! On stock wheels, 285/75/16's will fit easily, some have said that 305/70/16's will also fit. I have 16x8 with 25mm offset, they stick out further than the stock wheels - I had to trim the lower valance and the rear of the front fender to clear my 285's. I had my torsions cranked all the way up, but it was impossible to get my alignment within factory specs, so I had to de-crank it a few turns - brought it down from 2.5" over stock to about 1.5".
As others have said, you can just use a block with longer u-bolts to lift the rear, but I slipped some extra leaves in the rear pack to gain about 2.5". A block will give you the same ride as stock, while the added leaves stiffen the rear - it rides rougher, but it does give you more carrying capacity.
---------- Post added at 05:20 PM ---------- Previous post was at 05:09 PM ----------
I see you guys talking about turning up the torsion keys. This is a first for me. Where and how do you do that. It that's the case I will go with this method instead of buying a lift kit
Under the truck, about even with the rear of the front doors, you'll see a crossmember tying the left frame rail to the right. On each side of the truck, you'll see a long bar (the torsion bar) running from the front a-arm to the crossmember. Inside the crossmember, you'll see two bolts - these are your torsion bar adjusting bolts. I believe they are 18mm, and they should have anywhere from 4-8 threads showing if your truck hasn't been screwed with previously.
With your front end jacked up (NOT by any suspension points! you can jack from under the front differential), you can turn the 18mm bolts in a couple turns. Just make sure you turn both bolts in the same amount. Lower the truck, and drive a bit to settle the suspension in. Repeat as necessary, until you get the lift you want. Yes, you can max them out, but your ride will be rough, and may well be impossible to get it aligned correctly (see my previous post).
Then go get it aligned. With the unequal length a-arms, turning the torsions up plays havoc with your toe-in and toe-out, and your caster and camber. Don't expect your tires to last 40-50,000 miles without a damn good alignment. Also, the higher you go, the faster your suspension parts will wear.