My auto teacher said you have to change the front axle ring and pinion if you replace the rear, is this truein my case i have a Cherokee 4x4 and i need to get bigger tires thus the need for a different gear ratio, and would it be bad to run a 3.73 with 33 inch tires?
Assuming you want to run the front axle - YES your auto teacher is correct. Both ratios should be the same. 33 inch tires are not that huge. What are you replacing? tires that are 29-30 inch? 10 % is not so big. drive it the way it is. if it is too doggy then change ratios. if you are 3.53 now then going to 3.90 or 4.11 may be best especially if you are off roading.
The heavier anything is the longer the stopping distance.
You need to have the same gear ratio and tire diameter at both ends of any 4X4 vehicle. Taller tires usually require 'taller' gears to keep the overall ratio relatively unchanged and not mess-up the speedometer.
Your teacher is correct. You need to change both sets of gears. The front and rear gears need to be the same or the front diff will turn at a different speed than the rear and could dammage the transfer case. As far as going to 3:53 gears, with 33 tires your vehicle may not have the power to pull the larger tires, especially at highway speeds. You may need to run in 3 rd gear rather than 4th. For four wheeling the transfer case's low range may not be low enough for you to do the climbing you want. Your speedometer will be off as well, but you can change the drive gear in the transfercase, unless it's electronic then you might be able to adjust it in the computer. Former Auto Service Tech Teacher and ASE Master Tech 30+ yrs.
You are right. Brakes being equal the heavier it is the longer braking distance.
