OK -
Sounds like you might be ok the way you are...
It does not matter what your numbers are now...just get them as a baseline. (You might want to return to it, or get a wee bit of lift...your choice/no wrong answer) - measure at all four wells.
If you are ok with your present ground clearance (The trails you described...if you don't drag the bottom/hit stuff, you're OK)...leave it that way.
If you add the bumper, and it sags the front a bit (Smaller numbers at the wells after adding the bumper..) you want to crank the t-bars you already have up a bit, back to get the baseline well height numbers again.
The height of the truck will change your alignment a bit...so if the front sags, it will affect your alignment (Might not be a measurable difference though for just a bumper...)...but all things being equal...
You will maintain the original handling, etc...better if you bring the front end back up to where it was, if it had sagged.
(Before/after #'s tell you if it sagged...)
Cranking the T-bars is easy...anyone can do it.
Loosen the lock nut, crank the bolt head a few turns, retighten the lock nut...that's it. (Two of them though, one on each side)
The t-bars go from the frame crossmember (Where the adjuster and those nuts are...) into the front lower control arms (LCA).
They brace/support the (LCA) arm with tension provided by the twist on the bar...so as the wheel goes up and down, the bar is twisting and untwisting...its a spring essentially.
Twisting the bar more forces the LCA downwards, raising the truck's front on that side.
Adding weight can fight and win against the tension a bit, and just like adding weight can compress a spring, it can twist the bar more than is was, lowering the truck.
When you crank the t-bar back to raise the truck back where it was height-wise...you are adding some more tension/twist to fight the extra weight's resistance to the intitial twist.
New idea -
If you want another 1.5" of height than you have now, for about $50...
Order a longer set of rear shackles (The things the rear leaf springs are hanging from at the rear end of the truck...about $50.
When they arrive...
Do the t-bar crank like above...but give it an extra turn or so until you are about 1.5" higher on your well #'s than you were at baseline...
Install the longer shackles...and
Voila!
1.5" lift is done.
You should have an alignment done afterwards to restore the original handling feel/tire wear.
I've typically waited for new tires, as I typically have an alignment done then anyway...but aligments are not that expensive..so its not a big deal.
If you've never crawled under the truck before, and are unfamiliar with the parts...its a 30 minute job to do the t-bars, and an hour and a half for the shackles...because you'll be wasting a lot of time getting to know what's what..."wow, look at all that rust!" and "where's that wrench?" type stuff.
It takes me about 10 minutes to do both t-bars (combined), now...and about 30 - 45 for both shackles, mostly jack limited.
If you spray the involved nuts/bolts with a good penetrating oil every day for a week before you start...it will loosen the rust welds on the involved nuts/bolts...which takes more time the first time for that reason.