what rans to use
 

here's what I have;;;;84 chevy dulley with a 1986 industrial cummins eng[12 valve]have bell housing that accepted my 4 speed trans[chevy]and have a US gear vendor over/under aux trans;;first issue in gear vender went out of business and had my motor on overdrive stop working;;now my trans is making a growing sound[sometimes rear bad and makes howing noise when back off throttle in any gear..not sure if it is overdrive or trans but it's BUT HOWLS BAD WHEN COASTING AND ALSO HAD A BAD GRINDING NOSIE[SOUNDED LIKE BEARINGS OUT OF MANS;since not sure what one noise coming come so thinking maybe get a newer trans and do away with the gear vendor..what trans be closes for me to fit[

I can barely read this.

could be the ring and pinion and not the trans? R&P can howl during trailing throttle

not R&P;;;For sure coming out of trans;;Also can push clutch in and it stops;;most time it makes howling/grinding noise when I let off throttle but as soon push in on clutch it stops;;with the trans making noise and the over/under aux trans is in high side and can't take out of high side i'm thinking best thing be just replace the trans but wondering what trans be best to go with;;I tow a race car trailer[12000 lbs loaded]so need something that would give me close same mpg as I was getting with my combo I have now;

Not sure whether you have an automatic or manual. The 47RE automatic should bolt up but it has a lock up converter and shifting solenoids that work off speed sensors and a ECM.
The A518/46RE Auto may be the best bet for simplicity I think all it has is a neutral safety switch.
For a manual trans I would probably use a New Venture 4500 since its a 5 speed top loader and very reliable in most applications. If you have the cash a NV5600 6 speed would be good too.
I remember when if you wanted an overdrive trans gear vendors was the one too use and they where expensive.
Good luck with the swap and X2 on checking that rear dif like dieselXj suggested.

checked rear end and it's in perfect condition..I'd rebuilt it and change gear ratio couple years ago and only use truck to tow with in drag race season so positive no issues there;I have manual trans with gear vendor over/under in truck now;;the 2 trans you recommended are dodges;;I need chevy trans..if not chevy i'll have to change clutch and bell housing and flywheel..be a big expensive swap using dodge so want to stay with chevy;;Do you think my MPG stay same or close if I went to a 5 or 6 speed trans?mine now is a granny low 4 speed [don't use 1st gear at all.way to low..only move couple feet and already have to shift to 2nd so I take off in second and use my gear vendor in each gear;;