91 240 rear gears

Slight muffled clunk heard infrequently, 3-4 seconds after shifting into 1st and 2nd, from what seamed to be drivers side rear. Recently put in reverse and went about 6 inches. Could drive forward but reverse would lock up at the same 6 inches. Could go forward but wound up against a wall. Jacked up drivers rear wheel rotated forward freely . Would lock up on 1/4 of a tire rotation in reverse. Lowered and rolled back 6 inches ,raised and lowered then got tire to spin backward several rotations. Lowered rolled back 3 feet, locked up. Jacked spun and lowered. Rolled back clear of wall, put in gear drove home 70 miles without incident. Parked and started driving my 850. Got back to 240, put in reverse no problem. Put up on stands removed rear plug and drained gear oil. Oil looked clear, but magnet had about 1/8 inch of metal sludge and some small metal splinters. I can get a rear for $250 from a junk 91 and do a whole swap, but it's 0 degrees here, I do not have the time, and I would have to change all the bushings, springs, shocks, stabilizer bar if I had it all apart, and I can not afford that now especially if I have to buy the rear. I could get the rear and swap the gears but I'm not anxious to fool with pinions and shims and axles, and will it howl when I'm done. I could get the rear and see if the dealer would put it in, but that might be the cost of all the other upgrades I would like to do. (when it's warmer) Anyway, any posts anywhere of a rear gear swap, or rear end swap. Can I use a 740 rear, or the gears from a 740 or any other model. The Bentley Manual says I have to feeler gauge the speedo pickup in the housing, but not much about a removal and replacement of the gears themselves, except that you should leave it to the Pro's Thanks Ray