I've had my Superlift 6" for a year & a half now, and I love it! FYI here's what I posted after I got it:
FAQ's:
-The ride is pretty good, a little bouncier when lightly loaded, but that's probably due to the Superlift shocks and the big a$$ tires.
-Gas mileage seems about the same as before. I'll have to run a few more tanks of gas through it to see.
-Install was done by 4Wheel Parts. Labor was $750.
-Handles pretty good. Adding the rear spacers made it seem a bit less tippy. Turning circle seems to be reduced though. I'd like to see if I can adjust the turning stops a bit to ease parking lot negotiation.
-Acceleration is a bit slower due to the big wide tires, but nothing I can't live with. The 4.3 is a pretty strong motor! Highway power is adequate, downshifts a bit sooner on hills. Don't know what the stock gearing is now, but so far don't feel the need for regearing. Haven't taken it offroad yet, so I reserve the right to change my mind. Speedo is about 5 MPH slow.
-No rubbing at all with cutouts! I really want 15x10 wheels cause I like the look & the tires would probably wear better. Front turning clearance may not allow it without a body lift. I'll research further to see if it can be done.
-Step rails were a must for passengers, as well as pointing out the grab handle.
-Chicks dig it, & that's always a good reason to spend money
Only problems so far-The camber had to be readjusted after about two weeks, 4Wheel Parts did it for free. Also the torsion bars tend to creak, a shot of WD40 cures it for a while, but it's a reoccuring issue. Other than that, no worries & it looks great! Check out more pics in my gallery

The work was done in 2 days by my local 4Wheel Parts shop for $750 labor. I'd say that shop you talked to doesn't know what they're doing. Try another shop!
[size=75]2003 ZR5, 6" Superlift, cut out flares, step rails, 1.5" rear spacers, 15x8 Pro Comp 1079 wheels w/ 3.75" BS, 33/12.50-15 BFG AT-KO tires[/size]