I measured my old parts, and it seems your piece is not the jackshaft tensioner after all, but rather the balance chain tensioner.
The jackshaft tensioner metal strips are about 9/16" wide, about 3 3/8" long (if flattened).
The balance tensioner metal strips are just a hair under 1/2" (maybe 31/32"), and about 2 3/8" long (if flattened).
So your piece appears to balance chain tensioner (measure yours once more to be sure).
That tensioner has just two metal strips, so yours is down to one. You can replace just the tensioner without pulling the front cover, but if you want to replace the chain too (and gears of course), you'll need to remove the jackshaft chain (which means you need to timing aligment tools), so if you do the whole balance kit you might as well do the jackshaft kit since the parts are cheap enough.
To do just the balance tensioner (you won't need any timing TDC alignment tools, tensioner is < $20), you remove the front axle, remove engine mounting bolts, remove starter, remove lower pan & oil pickup, remove ladder frame bolts (some of these are tricky, especially at back of pan), jack up engine, optionally remove the drivers side Cat (won't have to jack engine as high), remove ladder frame.
gaskets are reusable. You might want to do the oil pump too since its right there. You need a 10mm Star socket to remove the two tensioner bolts.
Tools I know you need (may be more, just from memory):
- 13 mm socket
- 18 mm socket
- 10 mm socket
- 10 mm Star socket
- Torx bit - size? (rear ladder frame bolts, oil pickup)
- assorted extensions
Here is a link with details on those ladder frame bolts: (scroll down to 4.0 SOHC E engine)
http://www.autozone.com/az/cds/en_us/0900823d/80/0b/8f/f8/0900823d800b8ff8/repairInfoPages.htm