Was Jack in love with someone else right before his death?

(quotes from IMDB quotes)

Watching the film Brokeback Mountain, the love between Ennis and Jack can't be questioned. They are into each other in a major way. From relatively young guy's right up to middle age guy's. Meeting as often as work and life schedule will allow, even to the point of pushing the envelope and ostracizing themselves from their respective wives.

Like any relationship on screen or in life sometimes love just ain't enough. Ennis as we see, is never going to be able to cope or live in an "out" relationship and Mr Twist he has needs and a high sex drive. But try he (Jack) does to convince Ennis of life they may have: "Ya know it could be like this, just like this always. - Jack Twist"

Ennis counters always with words more of his time and upbringing than his heart: "Bottom line is... we're around each other an'... this thing, it grabs hold of us again... at the wrong place... at the wrong time... and we're dead. - Ennis Del Mar". Ennis goes on to somewhat explain his paranoia, further distancing himself from ever being able to fully be with Jack: " You ever get the feelin'... I don't know, er... when you're in town and... someone looks at you... suspicious, like he knows? And then you go out on the pavement and everyone's looking at you like they all know too? - Ennis Del Mar"

Ennis is even aware of Jack's sex tourism in Mexico and threatens to kill Jack if he found out the extent of it. Jack as always is persuading, trying to persuade Ennis to settle down with him " Tell you what... the truth is... sometimes I miss you so much I can hardly stand it. - Jack Twist"

We come to the nuts and bolts of it with the classic Jack Twist quote where he admits his sex drive, but ultimately in this monologue his (Jacks) need for male companionship " Tell you what, we coulda had a good life together! Fuckin' real good life! Had us a place of our own. But you didn't want it, Ennis! So what we got now is Brokeback Mountain! Everything's built on that! That's all we got, boy, fuckin' all. So I hope you know that, even if you don't never know the rest! You count the damn few times we have been together in nearly twenty years and you measure the short fucking leash you keep me on - and then you ask me about Mexico and tell me you'll kill me for needing somethin' I don't hardly never get. You have no idea how bad it gets! I'm not you... I can't make it on a coupla high-altitude fucks once or twice a year! You are too much for me Ennis, you sonofawhoreson bitch! I wish I knew how to quit you." - Jack Twist

This brings up to your quote: "Jack used to say ENNIS DEL MAR he used to say I'm goin' bring him up here one a these days and we'll lick this damn ranch into shape! Had some half-baked notion to build a cabin, help run the place. Then this Spring, he got another fella goin' a come up here with him, build a place help run the ranch, some 'ranch neighbor' a his from down in Texas. He's goin' a split up with his wife and come back here." This happens after the last time Jack and Ennis meet up and Jack did his darn'dest to convince Ennis to get ranching together, but as stated Ennis unable to commit and is also aware of Jack's being with other men. The closest we can come to a name is Randell the cowboy Jack meets in 1978 at a dinner soiree, although when swapping stories with Ennis before they part for the last time Jack says it's actually the rancher's wife he is involved with and we are left to read between the lines.

Jack loves Ennis for sure, but the years of living without what he most desires more than his wife - male companionship - is too much. If Jack can't have Ennis the one he desires most of all and who retreats more and more into the bottle, then Jack Twist being the man he is is sure gonna make sure he comes close to some kind of dream. We see this point throughout the movie, Jacks a dreamer, a risk taker, a tough man no doubt, but a dreamer nonetheless. Ennis is a whole other man brought up in harsher way and completely torn up inside over something he never entertained in his own mind till he met the person he truly loved, but still would never be capable of full time commitment.

Had Jack lived and ranched with another man, possibly Randell the rancher Jack meets at the 1978 soiree, who talks of the cabin and fishing they can do together you can bet when he was sitting on the porch of a night, whiskey in hand and smoke dangling from his lips, it would be Ennis in his mind and heart - Jack and Ennis's ranch not whomever happened to be sharing Jack's bed. This is perhaps backed up by the bloodied shirt of Ennis's that Jack stole 20 or 15 years prior and did not tell Ennis. Jack kept Ennis's shirt inside his own, a symbol maybe as at the end of the movie we see Ennis has placed Jack's shirt inside his own.