You're dead wrong. There'* a huge difference in 3.06 gears and 2.84 gears. If you don't listen to me and take it back, you will regret this for the rest of the time you own the car.

Every single owner of 2.84 gears on this Forum wishes they had 3.06.

Your trans shop is taking the easy way out so they don't have to replace what they screwed up. Don't let them off the hook. You are being abused in the worst kind of way.

x2 on what Wren'* saying. Your problems do sound like the wrong tranny for your PCM. I think we should make absolute certain of this.

Find the RPO code sticker on the spare tire cover in your trunk, and ideally post us a pic of it, or at least find the gearing code for us. It should be one of these on a '99 SLE n/a:
FQ3 - ratio, transaxle, final drive 2.86 (w/hi stall)
F83 - ratio, transaxle, final drive 3.05 (w/hi stall)

Then go up under your new tranny and ideally post us a photo of the tag or at least write down the first 6 numbers (going down, next to the barcode).

Pattern should be: "61yxxx"
y=year, xxx=3-digit gear/stall code

After the 61, it should be one of these:
8YFB (2.86 / lo stall)
8TPB (3.05 / hi stall)
9YFB (2.86 / hi stall)
9TNB (3.05 / hi stall)

where would the tag be on the transmission?

Near the steering rack underneath. You'll need ramps or jackstands. Tag faces the rear of the car.

The change in ratios will increase your torque by about 7% and decrease your mileage by about 5%, if you go back to the stock 3.05 ratio.

The higher stall torque converter will also increase torque and produce more power for acceleration.

Gears make it go.

Gears can make it slow.

well right now i cant do anything about this because i cant be without a car.And anyways in about a year Im going to be going to college to be a mechanic so after that I will be able to fix all this, i think the car will do just fine until then.

I've been a Master Tech for 8-9 years, and messing with 4t60-E transmissions is still something I avoid like the plague, they're kind of unpredictable and complex. I switched my 88 4T60 in my Lesabre T type from a 2.97 to a 2.84(because I found a good used SRTA free) and had no idea at the time what mistake I was making.

I tthink it'* a good time to stand up for the FWD GM cars and no let these guys just generalize your car like it'* any other old man Lesabre custom. It'* an special option package on your car, and you didn't ask them to throw your gas mileage or performance off, they just slacked, took the easy route and got away with it.

Make them make it RIGHT. And tell them to provide you with a car while they do. This is going to cost you in fuel for quite awhile. They OWE you.