Unfortunately I do not know my differential gear ratio. How do I figure it out? I am currently running 31 inch tires, and it isn't that its running poorly, it would just be easier on the truck to have the 5th
Overdrive and run at a lower rpm.
What I am trying to say is, if your Jeep is stock & came with a
T-4 it most likely came with a 2.73 gear ratio.
If I'm right it would probably not pull with a 30% reduction in gear ratio, so you spend money on a
Transmission that is weaker than your stock unit & you get "0" benefit from the expenditure.
That being said you seem to think you can make this swap without changing any thing but the
Transmission . If I'm not mistaken the
T-5 is longer so you'd have to a least have new drive shafts front & rear.
To address you gear ratio issue, if your rear is stock & unmolested.
Your Model 20 differential should have a metal tag: D=2.73,B=3.31,A=3.54,GG=3.73,C=4.10.
If there is no tag (old vehicle) not surprising you can put it on jack stands shift in to neutral, make a chalk mark on the drive shaft & the tire. Then rotate the tire & count the revolutions of the drive shaft to determine your ratio.