I would like to know what exactly are the differences between Panzer IV H and the Panzer IV J?

I know there were revisions in the exhaust system, as well as other modifications to tools and super structure.

Here is the special feature of the J:

1. elimination of the electric traverse and its associated auxillary engine
2. hand traverse was fitted
3. previous auxillary engine space was used to increase fuel capacity
4. Turret Roof armor thickness increased, and a 'Nahverteifigunswaffe' added as smoke projector and close defence.
5. During production run, pistol and vision ports for the turret rear and side doors are deleted.
6. Wire mesh skirting on the sides
7. 3 instead of 4 return rollers
8. Steel tyred road wheels
9. Vertical exhaust mufflers
10. Hull sides extended beyond the front hull plate, and drilled to provide holes for towing.

HTH.