To run 33x12.5 on a 91 you only need 2" front and 2" rear and a trim (1/2" to 1") of the front fenders.
My 86 (different generation, but similar suspension) had 35's on it for a short time this spring and cleared fine with a 2" spacer under the front springs, a 3" AAL and 4" cutout flares. The backspacing on the wheels were 3.5"(the half inch more spacing eliminated the rubbing on the inner fender on flex) Backspacing on the wheels will be the most important, since the wider tire/wheel combo will rub the inner fender liners on both the front and rear.
All suspension modifications will change the way the vehicle behaves, and all trucks are different. A 2" lift on your truck is different from a 2" lift on an identical truck. I have 4" lift coils on the front of my 86, but it only lifted it 3" from stock because of the weight of my bumper. My front end was sagged 1" to begin with :)
Results will vary...but basically a 6 or 8 inch lift is overkill for 33's...maybe settle for a 4" front and back, and make sure you get new shocks designed for that much lift...
There are a few members here from Jersey for example, with a lot more experience on Gen1 and 2 RBV that could probably help. Go to your USER CP and add a signature to your posts. Include specs of your truck, so your questions may be answered swiftly, instead of waiting three days...