Transmission

Nothing beyond 1994 but check with a junkyard or transmission shop.

Depends on exactly which transmission it has. If it has overdrive you can only use a 1993 or 1994 E4OD unit. If it has the non-OD then it is a C6 and you can use from 1992-1995.

Yep. The later trans would need the supporting electronics.

Make sure that your truck isn't actually skipping second gear and going right to third. Intermediate clutch failure on an E4OD can cause it to stay in first until third gear is commanded and then it will shift from first directly to third. Going directly from first to third will cause a harsh shift.
Pay close attention to the actual number of shifts to determine if this is happening.

If second gear has excessive wear or glazing it could also cause a bang shift. These conditions will cause it to slip as it shifts, then a hard apply at the end as the trans pressure rises to stop the slippage.

A problem with the 1-2 accumulator could also cause a harsh or delayed 1-2 shift.

Pretty much all of these are signs you need a rebuild.